I'd go further and say you shouldn't fly cessna singles either.
However, guessing, I'd say that by opening the right door you are creating drag on the right hand side. This tends to yaw the aircraft right initially, creating sideslip from the left (i.e. nose has gone left, so relative wind has gone right).
The C150 certainly has positive lateral stability, which means that it rolls into sideslip. So, it tends to roll left into the generated sideslip, caused by opening the right door.
But I am only guessing.
G