Looks to me like someone may have pooched gluing the surround down to the frame. Or the mounting hole has a bump in it. That part of your surround isn't "live" anyway and it's likely this would take years to fail if it would at all.
You can simply put your finger in there and see if you feel wood through the surround or if it's just the surround set on the frame a bit off.
*edit* if that sub isn't inverse mounted in the picture it's whoever glued the surround down. Once surround is glued to the frame it won't come off without an angle grinder so you're pretty much stuck with a dent in it if you do this while gluing it up.