Drunk Pilot makes a good point. Did the number one engine/prop come out of feather momentarily increasing drag just before increasing thrust and dip the left wing?
IF we didn't have the video, the solution might be more forthcoming. But the number two engine is clearly visible and its prop is spinning rapidly. The pitch of the prop blade is not visible however.
Having a cockpit video camera would show us what the pilots saw on their engine gauges, and how they responded to it.