Some people feel uncomfortable or awkward wearing formal clothes, especially if they don't wear them all that often. This would put one at a disadvantage negotiating, and it's not like he'll be kicked out of negotiations for wearing a hoodie.
This is just one possible explanation, but you shouldn't assume he's being a jerk for jerkiness's sake. Also, I'm sure whoever is negotiating with him is commanding a fat paycheck and used to negotiating through situations far more difficult than a dude wearing a hoodie. Bankers tend to just like to complain.
Finally, I'd be mildly surprised if you used underdressing to interviews as a successful tactic. (EDIT: underdressing means dressing lower than expected, not dressing down in general.)