May refer to the practice of setting a cell phone to "ring" not with sound but with vibration. Worn on the fanny (or inside a fanny pack), such a phone would be communicating an inbound call in "Ass Mode."
"Can't help it. I'm in (or: 'The phone is in') Ass Mode."
Ass Mode was first seen on the CBS "Late Late Show" with Craig Ferguson. Where Craig often say's something that is not politically correct or controversial ad will end the statement with the CBS catch phrase "CBS Cares" This behavior has also been seen on the "David Letterman Show" when Dave will say things about GE which owns CBS.