Lots of reasons. In a pro car, it's really rare, because they use them so often. In a sportsman car, it can be from lack of testing of the setup, lack of use for ages, mistake in packing from lack of practice, or an error in setting up the air system (as mentioned)... Lots of possibilities. It's a mechanical thing, packed, maintained, setup by humans -- therefore, it's subject to a lot of issues and errors.
In Seattle, they made everyone with a chute pull it during tech (first time I've seen that). A good buddy of mine had his just sit there when he pulled the lever. It hadn't been opened in years, and the mold/mildew had basically settled in and frozen it. He bumped it a little, and poof, out it came. Of course at 170+ mph and with a lot of wind/bouncing/etc. it would've come out. But scared him half to death, and he had to repack it and do it again to prove it to the tech guy.