As a recovering crazytown improviser, let me just say I feel your pain. It sucks to be in a scene where you are not getting laughs while making huge motions with your hands squawking like a parrot.
So, I'm going to echo a lot of what's already been said and add that in my crazytown express, the major cause is panic - letting fear get the best of me and my intelligence. The reasons for panicing can be many and varied, but the effects are not listening, getting in my head, invention, heightening details/emotions rather than game, embellishing rather than exploring, doing more than thinking.
It's like being at the front of a roller-coaster you can't stop.
The solution for me, was to drop big characters and big action at the top of my scenes and instead focus on listening. It sounds bizarre, but the cause of most of my problems were because I was so nervous on stage I would create characters I couldn't play and actions that I couldn't sustain instead of listening to my scene partner. My solution was to start small, establish the scene, and take my time.
I also am a firm believer that you have to train yourself to not to worry about the laughs. I think this can snap performers into panic mode. "Why isn't anybody laughing? CRAP!"
The good news is, crazytown is not fatal and there are plenty of teachers and coaches who can snap you out of it. Chris Gethard did wonders for me... although I still panic in auditions, I haven't panicked on stage in a long long time.
My favorite exercise for this is a Black Harold or Black scenes, where the express point is to perform totally seriously and go for the tragic side of scenes. Sounds strange, but being totally serious and dramatic was some of the funniest scene work I ever performed. This isn't the don't go for laughs exercise, but rather try to act your ass off. Once I realized I didn't have to put on a crazy character to get a laugh, improv was less frightening.
Of course, this could just be me.
