I see two reasons there:
1. BTC price reached $5000, which is a very strong psychological barrier. Many investors thus decided to take their profits at this level.
2. It might also be a typical weekend dump, abusing the fact that people can't deposit fiat money into echanges during the weekend. The dump triggers stop loss orders and price falls even lower. Speculators can then buy cheap BTC before the price goes back up during the next week, when people can send money to echanges again.
Yes, I think the reason for the fall is point 1, and then the effect of point 2 is to exacerbate point 1.