As I heard, they suspect a failure in the Autopilot for the problem. The official incident notification report or the Swiss Aircraft Accident Investigation Bureau states the following:
During climb to FL90, the flight was recleared to FL 100.
When entering the new command in the FGC, the aircraft
made an abrupt nose down movement and was difficult to
recover. In spite of pushing the A/P- and Trimmdisconnect,
the Pitch trim wheel was still turning NOSE
DOWN. After recovery, the aircraft was landed in LSZR.