Let's put this in perspective.
Traditionally a new administration always loose's seats in a mid term election, so it's a given some will change no matter what else is going on.
Second, when the economy is in bad shape, again, seats change. In fact, that's the #1 reason Obama was elected in the first place. Repub's paid a very heavy price for screwing up the economy. We could argue back and forth over who's fault it was, but the fact is, Repub's were in power for eight years. The economy got trashed, your fault, my fault, it doesn't matter, Repubs took the heat big time. Now with huge deficits as a RESULT of the economic crisis it doesn't matter if Dem's did the right the right thing or not. They ARE going to take the heat come election time. It's a classic scenario played out and over and over every cycle.
Third, a Demo majority WITH a Demo Pres is pretty rare as well! "Natural selection", if you would, always tend to put a balance back in favor. The end result often being political dead lock and they can't get anything done.
Defectors are rare but not unheard of, this is one guy who has been at odd's with the party for some time. It's not out of the blue.
All in all, just another bump in the road.