Car windows are much more better built for a variety of reasons, while bus windows aren't.

Car windows are designed to resist being pulled around without power for security reasons, the window tracks and actuators are very tight. Bus windows don't need that and are just tempered glass panes put on a sliding track or regular window frames.

Bus windows are also much bigger, their natural frequencies being lower and closer to the engine/road vibrations making them easier to go into harmonic oscillations when on the road. Perhaps the frequency didn't match when driving, but it definitely did when you lean on it like pressing the string on a guitar, making it vibrate with the engines.