Hi,
It is actually quite hard to explain.. What I can give you are some hints, some pieces of information that might explain partly what we can see..
First, we are at the end of the year. This means that the bitcoin rises generally speaking. This year, though, the rise was very impressive and particularly chaotic.
Then, there are people betting on other crypto-currencies that bitcoin, to avoid being stuck.
Third, there are some firms (CME and others) trying to make the most of such a strange market.
Finally if you add all this, we can imagine that after huge rises, people tend to be quite reluctant to bet again on bitcoin (because it seems to be "too much") and other crypto are trying to "win" market shares.
That's how I see things. I don't say that it fully explains the situation but at least it makes it logical..