In my experience, the main reason is any ad content you're serving. If an ad network's content times out (which I've seen happen for hours at a time, multiple times a year), your page might take 30 seconds until it fires the DOM ready.

At one company I worked at, the ad network we used got so bad that we not only executed all our JavaScript at the bottom of every page (instead of in the DOM ready), but also only loaded the ads after that, and then dynamicaly repositioned them to the "inline" spots where they supposed to appear.