The cause of this, ironically, is a GM trying to keep the game fun and interesting. It is a mistake born of good intentions and the mistake that sometimes less is more. To get more tension in some moments, it needs to contrast against moments where there is no tension. In short I'm talking about necessary mundanity, just like others may talk about necessary evil or necessary bad weather. After all, if it is sunny, 74 degrees, and clear skies every day then what is the significance of that beautiful day? Nothing, it is just a day. Now add that day to a week/month/year that also has rainy, cold, and miserable days and that near perfect day can truly be enjoyed for what it is.
So, lets look at some things we can do to add some "mundanity" to the game, or at least take away the tension for a bit. That way, when we do add the tension we can really enjoy it. After all, it will mean more as a spike instead of just a rise in tempo.
Don't Roll For Everything
Most systems have suggestions for this somewhere in the GM section, but honestly you don't need to have your players roll for something. If they have a clever idea, and the problem isn't truly important, just let it work. If Joe the Hacker wants to crack into an ATM for some extra cash, why not just let him? Joe bought those skills, he's one of the best hackers in the world, is an ATM really something worth slowing things down with a roll?
Even better, just letting players do things (provided their character is good in the area) lets you simultaneously lower tension while enforcing that they're good enough. "No need to roll, you're hacking skill is good enough to just do it" not only speeds up the game, but it also tells Joe's player that he is as good as he may (or may not) think he is. Even better still, since Joe is good enough to just hack through the ATM (or several other things) when he does have to roll, then he knows it is a bigger deal. It is a challenge to even his skills. Play this up and by lowering the tension by hand waving some things, you increase the tension when you don't hand wave it. At least, potentially, a roll the player breezes through is still just that.
I'm just going to the store...
I was in a game once where everything, literally everything that the players tried to do turned into an adventure. To the point that just going down to the store to grab some dinner for everyone turned into a fire fight, because a group of 6 armed (well armed at that) thugs were trying to rob it and the PCs when we showed up. Now this can be fun at times, especially with the absurdity of it. But seriously? Why couldn't I just go to the store, grab some food, and go back to the base so we could continue the main story?
The tendency to make everything an adventure I think comes from television. After all, there are regularly entire episodes of shows dedicated to just that. Tommy goes to the store, and the store is being robbed which turns into the focus of that show. It isn't necessarily a bad thing, but when it is happening constantly you have problems. Sometimes a trip to the corner market for milk and eggs is just a trip to the corner market for milk and eggs. So why not let it be that? I'm all for players being trouble magnets, hell a lot of times they will cause their own problems, but sometimes it goes just a little bit too far.
If you find yourself wondering why you are five to six sessions behind where you thought you'd be, look over what is happening. Are all these side things that are going on from the players or from you? Could some of them maybe be reduced in time by not being so exciting? Does your game lose something (aside from more chances to roll dice and shoot people I mean) if you stream lined these things? I bet the answer is that you can streamline it. The question then is, do you want to?
I think the important thing I need to say here is that if players are having fun with how things are going, you should by all means keep it up. After all, we are gaming to have fun right? However, if your players are starting to roll their eyes when things come up every time they try to do something, or are starting to pack assault rifles to go grocery shopping in a "real world" game, you may want to consider toning things down a little bit.
Also, there are some games where you don't want to lower the tension. Where you want to keep building and building up to a near breaking point. You need to be careful with that too. If you have good players who empathize with their characters, you can start giving the players quirks. I've given players nightmares in games by the fact that the session was 6 hours straight of high tension. I know people who have had players start exhibiting signs (thankfully slight) of PTSD because they kept the tension in their sessions too high for too long. These may be rare cases, but when players get into their characters, it can happen and you want to be wary of it.
There is a reason we have the low tension moments, and tension breaking moments, in horror and thriller movies. It is to give the audience a rest, a chance to catch their breath and collect themselves in anticipation of the next spike. You want to try to incorporate that into your games if you can. Give the players a chance to breathe, even if it is just a 5 minute "drink and bio" break in the middle of the session. Also, if you are going for high tension, you need to talk to your players and make sure they are having fun. Then again, you should be doing that anyhow.
Hopefully this helps with giving a way to create some mundanity and handle some 'boring' situations without responding with action sequences and dramatic moments. Spikes are good, but they're more fun when you have lulls to look back on them from.
Note: I put the story design tag on this because, while focusing on the idea from a game craft point of view, this is also important to keep in mind for story telling. Give breaks from tension to let people enjoy the next bit of it. Show a character being capable to set up a challenge as larger for later. All of this applies to story telling in general, not just running games.