Very interesting! This one finally answers my question!

In summary:

(1) It's not an American thing at all. I was wrong.

(2) It's not new. I was wrong.

(3) It has its roots in authorities charging or threatening to charge someone with the act of trespassing. I learned something today :-)

TLDR: "I was trespassed" means "I was charged with the crime of trespassing" (or "threatened with being charged with the crime of tresapssing").