This may or may not be the reason. It was due for a correction after an insane year of growth (all crypto, not just BTC). On top of that traders and whales could be taking profit for taxes. It's also bonus season so whales could be pushing down the price to get Wall Street money in and to buy back and a lower price (crypto is highly manipulated).
So what should we do
+Mike Anaba: What you should do is realize that past performance is not an indication of future performance in any investment medium, including non-physical commodities such as cryptocurrencies.
Have a diversified portfolio. Accept that all investments involve risk. Do not make investments using funds that are necessary for other purposes.
I agree with +Adam Alexander. Crypto is exploding but it's still highly volatile and prone to swings (and manipulation). Some provide real world use and are very cheap (Sia) others are not even decentralized crypto (Ripple – why people are nuts over this one is beyond me; yes I trade it though), others do NOTHING yet, and yet others solve real world problems (like privacy… Monero). Only invest what you can lose and realize it could moon or crash at any point.