RE:ReasonsOne of the more popular algos used by momentum investors/funds is the 10/100 Moving average. They might tweak the settings slightly, but overall, the strategy has proven to outperform the market.
Basically when the 10 day Moving average approaches/crosses the 100 day, it is time to either buy , if its on the way up or sell/short if its on the way down. The strategy is detailed in "Following the Trend: Diversified Managed Futures Trading" by Andreas F. Clemow.
I belive momentum investors/funds might be heading for the exits and this makes "small" investors sell. They will be back in once the trade heads the other way.
All the best,