People keep assuming the RD went into the wheel. The OP says clearly that he was in a mid range gear, so it's extremely unlikely that the RD went into the wheel before the hanger broke. Something jammed the RD or chain, and the RD was pulled back by the rider, snapping the hanger.

One telling bit of evidence may be the hanger itself. If the the break is a ductile failure, showing some bending before it snapped, then we know the jammed RD broke it. OTOH- if it's a brittle failure (clean fracture line, with no hint of prior bending) then it's possible that the hanger broke first and that was the cause.