The housing crash in 2008 was caused by many people getting kicked out of their homes. It was too easy to get a mortgage that was too big, so lots of families were one hardship away from being unable to pay their mortgage (economic calamity). Also because it was so easy to get big mortgages, housing prices were higher than they would have been otherwise (bubble). So after the crash started, there were a lot of big empty homes and few people could afford them since lots of people were getting kicked out. So sellers and banks had to lower their asking prices until someone could afford them. There are more requirements for getting a mortgage now after 2008 so that should hopefully help to make the next crash not as bad.

Basically, if a lot of people buy houses they can't really afford, a crash will happen soon: when all those people get foreclosed on in large numbers because they can't pay the mortgage.