Coinbase is as crooked as a barrel of snakes. Whenever anything semi-significant happens in the crypto-market regarding any of their coins, they will go down quickly. I can't count how many times I've looked at the charts, saw an opportunity, and was thwarted because coinbase was "mysteriously" undergoing problems with the very coins I was looking to manage. Unfortunately, using coinbase is normally the most convenient option for those of us in the US. However, the lesson is: do not rely on Coinbase for anything in an emergency or obvious money-making situation. If there is an arbitrage or buy/selloff situation that is obvious to the public, they will either be shut down for maintenance or extremely slowed down, potentially fucking you over even worse by holding your money in limbo during the situation.
Unfortunately crypto is not yet built for mass scale trading, be it by design or just young age. Either way, for your sanity, never rely on coinbase. Wait it out, or have another backup option ready.