Why is it that villain strongholds always seem to fall apart once the villain is defeated?
Glad you asked that, chum! Any hero who studies advanced villainous biology can tell you that a villain's heart is made of pure evil. I won't bog you down with the physics of it all, but suffice it to say that all of this evil makes a minature black hole that powers the dastard's dastardous deeds. That black hole is very delicately maintained by the evil thoughts of your typical do-badder. Once slain, that black hole runs rampant and completely destroys any nearby evil strongholds. It's just basic science.
Thanks for your question!
Well Henry, imagine that you were an evil no-goodnick. Imagine you spent your entire life amassing power, wealth and the blood of virgins. This is your life's work and you don't want some hero taking that away with them (particularly the virgin blood, that is worth a fortune on the black market). The typical evil maniac's way of thinking goes along the lines of, "If -I- can't have it, then no one else will!"
If you consider the mindset of your subject, then it stands to reason that someone like that would booby-trap their homes to destroy their ill-gotten gains as well as any heroes that might be in the vicinity. That said, always go into an evil lair with the expectation that it will fall down around you at the most inconvenient moment. Remember the layout as you go and try to make note of alternate emergency exits. A little planning could save your life!