To put it simply, the price goes up when there are more buyers, or down when there are more sellers. When there's been a heavy period of buyers (such as December) then the reverse is then expected (such as January), although sometimes triggered by "bad news" like back in summer with China regulation.
In this case, there doesn't appear to be any real bad news, it's just been correcting because there was too much good news.
We only eventually saw cracks at the peak of $20k when there were flash crashes due to "too high" volume on weak infrastructure from trading exchanges, GDAX notably. Funny thing is this is also what initiated it's down fall, Bitcoin's infrastructure wasn't able to keep up with demand.
This is probably why from $10k people are still very bullish on BTC, as I am. Nothing's changed.