According to Moody's, the downgrade and ratings reflect "the deterioration in the bank's financial profile and the significant challenges it faces". Adding that it is maintaining ratings under review for a downgrade as well. For some context, a drop from B2 to Baa1 is straight up putting the credit profile of First Republic Bank into speculative and high risk territory.
There really should be ratings agencies for ratings agencies themselves at this point. They have been so out of it ever since the events leading up to the global financial crisis.