It does look unintentional. Possible aileron stall?
You have a total of 60+ feet of aileron on the DC-3. Aileron stalls can be pretty nasty.
You can actually steer the aircraft left and right on the runway just by giving aileron inputs - Buffalo Joe showed me.
I'm familiar with a case where during an approach to stall the aircraft shuddered once then rolled inverted - the ailerons were not rigged correctly and one stalled before the other.
I've had the control wheel ripped out of my hand after landing due to an aileron stall with some ice on the wing leading edge.