Not sure why they move in tandem, maybe because their all the same asset class and the market sentiment is that people are still weary?
As for who makes up the majority its going to be hedge funds and mutual funds. Before all this cracking in the wall that happened as of the past couple of months, hedge funds use to control something like 80% of the volume, but only 20% of the money. Whereas mutual funds controlled something like 80% of the money and only 20% of the volume.
Those %'s are probably rounded out, but I read it somewhere in like the Economist a year or so ago. Might have changed, but it gives you a general understanding.
Same old lesson: Don't fight the trend.