Why level off prematurely would be more to the point. Could this not be the first in the causal chain. Given what appears to be on the face of it, a big heave- ho to arrest the climb in a hurry, an evasive manoeuvre, what could they have possibly been trying to avoid. I would suggest heavy weather seems the most obvious.
I don't think he was trying to level off, I think he either had, or perceived he had an excessive pitch attitude. Whether that was because of somotogravic illusion or an actual high pitch attitude or a combination of both who knows. I think his goal was to remedy a situation of high pitch while in a state of sensory confusion.