George Zimmerman fired the security company he hired to protect him -- the company that's now suing him -- because of mysteriously high billing ... this according to Zimmerman's lawyer.
As we previously reported ... Associated Investigative Services filed a lawsuit against Zimmerman ... alleging he still owes $27,027 for its services.
However, Zimmerman's attorney tells TMZ his client felt "the billing got out of hand quickly. We trusted them to be efficient with their work and they weren’t." He claims Zimmerman dropped the security team because of concerns about the bills.
The attorney adds ... Zimmerman's supporters were paying the tab -- through the George Zimmerman Legal Defense Fund -- and he says, "I don’t believe those who have contributed to help George should be taken advantage of by such over-billing.”
We reached out to AIS about Zimmerman's claim -- so far, no word back.