On a Dutch news site here (
Inmiddels 77 lichamen uit ) the following claim is made (translated into English):
The pilots wanted, thus becomes clear from the 'current conclusions' from the investigation into the two black boxes of the plane, to fly over a lightning storm, but were prevented from this because it wasn't possible due to the temperature to fly to that height.
Reading this kind of surprised me, as I didn't have the impression that they wanted to fly over the storm, but it would explain why they suddenly went into this steep climb.