["Almost every night, something keeps dumping over my big trash can outdoors. I keep watching for animals but the way its propped up on a board, I don't see how an animal can do it."]
I HATE when that happens. In my case it was a dog, I caught him on camera! Maybe you should invest in a security cam?
Almost every night, something keeps dumping over my big trash can outdoors.  I keep watching for animals but the way its propped up on a board, I don't see how an animal can do it.