Hi Belgique, I read your link. Modern autopilots leave you in a trimmed state as they disconnect. I'd be surprised if this wasn't the case with the Q400.
If, as you suggest, the autopilot disconnected because it reached the pitch trim limits firstly I'd be worried that the autopilot had insufficient trim authority to hold the aircraft in level flight at 134KT, which seems unlikely. Secondly, if that was the case, the aircraft would be trimmed nose down at disconnect, having failed to trim nose up sufficiently for the speed/thrust combination. Am I missing something?