Take a look at this article that describes how companies are borrowing money to buy stock in themselves in order to push up their own share prices: http://www.washingtonpost.com/business/economy/companies-tur...
Interest < Earnings * risk
Not surprisingly, when interest rates are low, companies borrow more money to do this. It's also a feedback cycle where companies pay higher prices to buy their own shares. But it's still worthwhile because money is cheap.
Thus, answering the original question, a market crash won't likely happen until interest rates rise. Recently there have been data that suggests the Fed might be able to raise rates, thus the mini sell off. But there is nothing solid yet. It's not the big scare, but the environment is building that there could be one. Doesn't mean it'll happen though.
That (and/or paying a dividend) is just what public companies are traditionally expected to do when they have extra cash, (or when they can borrow cash very cheaply, as is also the case now). Companies have basically been hoarding cash since the last recession, I think I've read that Fortune 500 companies' cash reserves are the highest they've ever been, so it's good to see that they are returning some of it to their investors now.