I'm not sure if enemies can pile on top of each other. It might just be the player.
On another note, I have realised that the custom falling animation is necessary, as you can jump off things like the Death Cube to get on robots.
Ah, now I know what you mean. Yes, they're a bit daft when it comes to getting back up the ramp. This is balanced out by Spidertron in the later tiers, but I agree it's pretty easy to lead them around for some quick kills. - I'm not quite sure if it's a bug, but I'll make a file for the AI. Thanks for the report!
-Cheers