Theory of supply and demand. Generally, as bitcoin demands keeps increasing and the bitcoin can't have difficulty supplying it, then the price increases. And then there's the positive news coming from different sources. This makes people go to the market and buy bitcoin resulting to the price going up. And vice versa, if there's a bad news, it cause the price to go down drastically.

Another reason is manipulation, there's a unwritten rule that whales do sometimes want to manipulate the price is either direction. Remember they can only do that because they have huge amount of bitcoins to begin with. It maybe a individual or group or individuals or sometimes financial investment company.