This could not possibly have been an accident.
Here's how it works: when Bowser bounces into the air, he is still in the ending animation of Bowser Bomb. However, this animation finishes a bit after Bowser hits the peak of his bounce, and Bowser is left in midair, facing backwards.
Since the animation is over, Bowser can do anything. However, since Bowser is only a couple frames away from hitting the ground, most of his moves won't come out in time, so it feels like he can't do anything.
Bowser Bomb halts Bowser's downwards momentum, which means (as AGS discovered) you can use it even if you're only two frames away from hitting the ground. However, the move has no hitbox during its startup animation (nor during the bounce), so it's even more punishable than ENB said. Also, the hitbox is a lot smaller than the auto-snap radius, so it won't be able to catch the majority of recoveries. (It'll meteor them if they stay on the ledge long enough, though.)
You certainly can use an aerial and have it autocancel, though this is no faster than just landing normally. Remember, "autocancel" means that the move cancels into Bowser's normal four-frame landing animation, which is the same animation that would have played anyway (because Bowser is in his normal air animation just before touching down).
Probably the most useful thing you can do is midair jump. Bowser Bomb gives you your jump back, and using it allows you to skip the four-frame landing animation. Then you can either get out of there or use an aerial.
