Man, this scene keeps getting more and more confusing.
First it seemed simple. Joker and his goons crash a bus into a bank, rob the bank, and probably escape in the bus or a second car. Then we found out about the zipline. Okay... so someone makes an escape by zipline. Wait, no. The zipline is going in TO the bank, not away from it. Then we find out the bus is parked facing outside, like it was backed into the bank, and that the Joker and his gang arrive in a suburban.
So... Joker and the gang show up in a suburban, more goons repel in, possible more goons back a bus into the bank for escape. Then Joker, and possibly the goons, escape in the bus by merging into a line of buses.
The question is, who's driving those buses? Did he schedule it to coincide with a time he knew there'd by lots of buses driving by? Maybe that's why Joker's the only one in the bus, because the rest of the goons went to go grab the other buses.
Damn you Nolan! It's all so confusing!!!