Why did I get asked to leave my single backpack behind upon entry of the bathroom, is this standard procedure? Is there anybody else who finds this unfair and possibly invasive of there personal artifacts?

It's because people before you have ruined it.. it's not you it's the people before you. It is not normal practice..

That is an odd policy. I have only gone in once. Usually use the drive through. Wonder if it is because on all the kids from the high school.i would check with the manager.

That is an odd policy. I wonder if it has something to do with use of narcotics and other controlled substances in the area? I would contact management to have them explain the reason.

Because establishments don't prefer to have people overdosing in their bathrooms so they just tell everyone to "please leave the bag behind".