Why are some porta-potties shaking at festivals?

I’ve noticed that at festivals, people sometimes have to wait in long lines for the restrooms, but some porta-potties stay occupied for a really long time. Sometimes people get impatient and even kick the door.

What confuses me is that occasionally the porta-potty is literally shaking 😭 I’ve always wondered why some people take so long in there, and why the whole thing moves sometimes. Has anyone else noticed this before?
Why are some porta-potties shaking at festivals?
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