[The Red Zone]

This bus shot would be something new. He can't set up an aim in advance for a bus door shot since he does not know precisely where the bus will stop.

[Oldeconomybuyer]

MSNBC is carrying a local TV feed. Reporter said this stop was a "turn around" stop and could have been parked for up to 30 minutes.

[Catspaw]

He'd get a general idea of where the bus would stop, and most buses will stop pretty close to the same place every time. This is a layover stop for the bus. The bus would be sitting there for a few minutes, not just stopping, taking on & letting off passengers and pulling out. All he had to do was wait for the bus driver to open the door, stand up and start coming off the bus to present a target.

[ARCADIA]

This bus shot would be something new. He can't set up an aim in advance for a bus door shot since he does not know precisely where the bus will stop.

Good point. Also, note this from AP

"The bus was parked at a staging area where drivers get ready for their morning runs, state police spokesman Cpl. Rob Moroney said. He didn't know if anyone else was on the bus."

It seems to me that someone needs to know this staging area exists, that it isn't just scouted like the gas stations near the highways.