The MOSFETs could have been degraded and finally failed. All in all I don't think the problem is any bigger than that. SMPS are current limited so the chance the load causes an SMPS to fail this way are practically zero.
I agree, it's rare to find a well designed, decent quality SMPSU that kills more than itself or that was killed by something on the secondary, besides, it's 'only' $200, taking a chance on that being a waste of money is far better than hacking out several thousand $ on a new 'scope.
Definitely worth taking a multimeter to the power supply connections to make sure there aren't any shorts though.