Sure, the MAX ALT given by QRH incorporates some margin.
What I wanted to point out is that they were most probably at the limit for normal ops. In my mind I do add some turbulence, AP loss including stall protection loss and some techincal problems to take care for (and to subtract attention) during a stormy night. I think every pilot would be at his / her utter limits.
As a probable cause I don't need any lightning or bomb, just too less margin with unexpected trouble, causing distraction.