Not a current employee but did work at Pandemonium two years ago. I'm honestly kind of at a loss as to how Panda managed to valley... I wasn't even entirely sure that it could during normal operations (believe me, we tried pretty hard to get it to valley during safeties that year). What were the weather conditions? I know it has some issues in wet and/or cold conditions, but that's usually fairly easily rectified by placing the ride in Weather Mode; even if it wasn't, it should still have been more than hot and dry enough to not cause any issues.
My only other explanation is a bit of a reach, but it would have to do with the power outage (if they are even related). If the power outage happened right as or right before the car crested the lift, the lift would've begun to slow down as it stopped, but still should've had enough momentum to allow the car to continue on the track. I suppose it could be possible that it slowed the car down just enough (with the right amount of weight/weight distribution from the passengers) to fall just shy of cresting the first drop before Brake 1.
If it is indeed related to the power outage, then it's just some bad luck; a few seconds earlier or later and they would've either stopped on the lift or in Brake 1.