Hello. New to the forum. Love the discussion and expertise. Question re: a rust hole I plan to fix.

I recently purchased a 1989 240 and have begun the journey to bring it back to life. I'm planning on a little paint/bodywork, including repairing this rust spot. I'm curious if anyone can tell me how this particular rust spot came to be. Is there an underlying water/drainage issue I should be trying to uncover in addition to the rust repair? Perhaps with the windshield? It just seems like an odd place to rust through. In general, the rest of the car is rust free (rockers, wheel arches, etc. - all super solid). I'm confident I can fix it properly...just wondering if it will be back.