A321 pilot reported during preflight the engine started when the APU bleed valve was selected on. Maintenance found the engine start valve was stuck in the open position.

Narrative

We arrived to the aircraft and as we were performing our flight duties; a Flight Attendant informed us the cabin was very cold. Boarding was to start shortly so I elected to turn on the APU. Once the APU was running; I selected the APU bleed air switch to ON. Shortly after I pushed the APU bleed air switch; I noticed that the number 2 engine start sequence had initiated. I re-checked the Engine 2 master switch and it was in the OFF position. I then noticed N2 on engine 2 was starting to rotate so I immediately shut off the APU bleed air switch. That caused the engine 2 start sequence to discontinue. I called maintenance and after some troubleshooting they concluded that the start valve for engine 2 was stuck open; causing the start sequence to initiate when the APU bleed air switch was turned on. This was a pretty alarming event. There was a fueler near the aircraft when the engine start sequence initiated. Fortunately no one was harmed during this event and the remainder of the flight was concluded with no further issues. I'm not sure how it could have been prevented as there was no way to tell that the start valve was stuck in the open position prior to APU bleed being selected ON.