I highly doubt it's incompetence - they're trying to perhaps single-handedly prevent a crash by everyone rushing to sell at once, to block people's impulse decision to sell for long enough to help further subside a crash. Coinbase's behaviour is highly suspect and perhaps should warrant investigation into purposeful manipulation - whether there are laws that currently exist that would cover this case or not doesn't mean there shouldn't be.
At this point it's basic incompetence, they have no excuse.