Not surprised. I think bitcoin's media attention and popularity was at it's peak on December; having frequent news about bitcoin and it's ATH price. Probably caused a lot of people to register on Coinbase and buy some crypto.
with higher price, higher the revenue, it is that simple...
in december the hype was really big, will see results for january/february
I don't think it's due to the higher price, probably more of higher volume. I think their percentage cuts on their buys and sells are constant, so they'll be making the same amount of money whether bitcoin is as low as $1,000 or as high as $20,000. All depending on volume/amount of trades.