You should hear me when I ask questions!

It is pretty much all of the things you mentioned. In a perfect world with no motor and absolutely dead straight surfaces the airplane would probably stall straight ahead. Put a running motor on the front of the airplane and the propwash ensures that the perfect world is gone. Add some asymmetry due to building and hangar rash and you get it falling off on one side or the other.

Why then to the left? I think the spiral propwash tends to raise the local angle of attack on the left wing and lowering the angle of attack on the right wing just enough to cause the left wing to stall first and the airplane to break to the left.

It could also be due to the fact that Spad airplanes are ugly.