In a bull market, you expect smaller-cap stocks to rally farther. In a bear market, you expect them to decline farther. The "generals" are less volatile than the "cats & dogs".
Trade it a bit and you'll see it often tends to overshoot the others. It's just its character. You'll begin to like that after awhile... Me, I am waiting on today's gap up open; remember Mr. Market, I said gap Up open...
Ok, I thought maybe it had to do with the debt market making it harder for small companies to borrow.
You could have noticed something in the recent weeks leading up to last on the RUT. When the 3 other majors were pushing up hard, RUT stalled consistently at 855.
Since it is small caps, they are a more representative index for US stocks. The SPX has a lot of international exposure. So if you want to short the US stock market, you'd go more with RUT than SPX.