There have been discussions about this suspicious behavior by Coinbase. While the official reason seems to be "bad servers" which cannot handle the traffic when people start selling or buying actively, it does not seem to be all the story here. Indeed, it's a big company with tons of revenue, and the issue happened multiple times before, so surely they could fix it if they wanted to, right? Some speculated that Coinbase might be doing it on purpose because they don't want to risk losing a lot of money if people do panic selling. Others pointed out, however, that it doesn't make sense that Coinbase would actually profit from transaction fees if they didn't close the market. All in all, it's shady and weird.