What's wrong with my buzzer driving?
Recheck all your wiring or board for shorts. If you can, start from scratch. If you manually pull the input high and the others low, does it still happen? Or only with your microcontroller?  If only your microcontroller it's likely a code issue. A ULN2803 is a simple transistor pack and would not experience these issues unless there was a short or code problem.

Also,  you should actively pull the other outputs low.  Do not leave them floating or as inputs.