This is basically the same glitch as the "Donkey Humps a Metal Donkey" in Jungle Hijinx with Rambi, except you do this one with Expresso at Orang-utan Gang...
There are a couple notable differences with this particular glitch compared to the Jungle Hijinx one:
1. If you continue to hold the Y-button after you throw the steel keg, Expresso will "hold" the keg while you're riding him. The Rambi version of this glitch does this too, but Rambi will destroy the keg before you get much of a chance to carry it around. Also, you can touch enemies with the keg, and it'll kill them (and destroy the keg.)
2. The Donkey/Diddy that you ride after releasing the Y-button is a different color than the one in Jungle Hijinx. It's probably because of a combination of the weird palette effects in Orang-utan Gang and Expresso's different palette from Rambi's.
PS: Don't ask why I decided to attempt this glitch on this tiny little ledge when I could have used the trees to the right.