Hmmmm, well that's interesting. Perhaps raids are based on times of most cell activity, similar to how spawns are influenced by cell activity. Maybe a high presence of high level players would spawn more level 4 raids?
Also a side note: Didn't Niantic state that pokemon in gyms would be returned to their trainers once a raid started? I've never had that happen. I was thinking that that would prevent stagnation and get some players their coins.