Couldn't it be as simple as the fact that the middle class has less ability to whether the storm?
It could be, yes. It could also be that the middle class is more vulnerable because the upper class tend to act in their own self-interest instead of the interest of the economy/nation as a whole --- for example, laying off employees while maintaining their own salaries and bonuses (we've seen lots of examples of that in the credit crisis).
Both of these explanations assume that crashes cause increased wealth inequality, but don't explain why booms create decreased wealth inequality.
A third possibility is that increased wealth inequality causes crashes. The theory here is that since the lion's share of consumer spending is driven by the middle class and below, decreasing the amount of money in that group of people will lead to decreases in the amount of spending and a crash. (This has the advantage of explaining the connection with booms as well.)
Which is why I said that the cause/effect relationship was unclear.