Whenever there is a big pump or dump in bitcoin prices, the coinbase crashes. Maybe because they are not able to handle the increased number of people accessing the site. They shoud upgrade their system. I don't see any other motive for coin base to purposely down their system.
I have noticed it also but I doubt they were doing it on purpose too, since we are having a massive speculation on the cryptocurrency market I guess they are experiencing some heavy traffic on their platform even more these days that coinbase and binance are the best performing exchanges. But if it turns out to be true that they were doing it on purpose, then clearly are manipulating the price of bitcoin avoiding it not to lose value because of the dump. But then again Coinbase is huge enough to put their reputation in expense.