Market bottoms usually occur when traders positively loath a stock/commodity/asset class. I mean that the "conventional wisdom" becomes such that no one pays attention to it any more - partly because the market participants were burned in the stock/commodity/asset class in question, and there's something new, shiny and much more profitable attracting their attention.

I'll give you some examples:

By the late 90's, the big players had pretty much written off ag commodities. They got burned in the 80's, the returns in the early 90's were mediocre, and by the late 90's, internet stocks were the "new hotness" where everyone was making big bucks. So commodities were pretty much ignored, setting the stage for the bull market run that started in the mid-00's, when stocks started establishing a pattern of going nowhere.

Then came the housing collapse. Stocks collapsed into March of 2009, but bonds were on fire. Bonds were on fire clear through 2010, and now bonds are looking like they're topped out for awhile. Stocks went on a tear last year, even as commodities were putting in price action that looks very much like they're going to sell off after a top.  

Ultimately, market bottoms are formed because panic selling cannot be maintained for months on end, or because false optimism has been crushed quarter after quarter to a point where some people just give up, sell out and go away defeated.