I've been thinking this comes from "makeru," which is "to be beaten (in a sporting event)" or "to lose (in a sporting event)".
I understood "maeta" to be s shortened form of "maketa", meaning "I lost" (or "you won").
In fact, I think I remember one of the guys telling me that once, a long, long time ago...
No, definitely not. "Maitta" has nothing to do with makeru; other than for tapping out on the judo mat, the verb maeru is also used for regretting a mistake or apologizing. Not the same as makeru at all.