What causes drive belt to come off?

It looked aliigned correctly. What the best method of checking this?

It's a 6 rib belt how tight should they be can it be too tight?

The idler was a little noisy the bearing somim getting a new one but didn't think that would be the cause

If all pulleys aren't aligned correctley they can work their way off check them with a traight edge. A worn bearing may also cause it if the pulley is oscilating