As for possible motivations --- aside the general DOS in progress --- it's quite possible that this is an attempt to exploit the "database failure leads to compromised DB credentials in the debug output" scenario that has been around for the last few months.

@houston: Someone did mention an umphy botnet at their disposal a while back, yes. Might be a good idea to set up a separate domain/server(s) for redundancy to take over when one server is down. Just set up a script for checking see more current DDOS targeting and rotate servers automatically as necessary.