There is a potential reason: There is "drag" resulting from tides whenever a body is rotating faster or slower than the orbital period. (First approximation: a

There is a potential reason: There is "drag" resulting from tides whenever a body is rotating faster or slower than the orbital period. (First approximation: actually there are weird things like orbital resonances. Mercury for example.) This drag becomes much more prevalent when the body is asymmetrical - say, when a large body just hit it. (Again: first approximation.) As such, one could conceivably say that the moon was rotating faster than it is now, and the impact slowed it down enough that the drag in the meantime was enough to tidally lock it to Earth sometime between the impact and now. (Or slower, and sped up. Same thing.) Although I don't know nearly enough to know how large the "window" would be of rotation rates that would tidally lock in the amount of time between the impact and now.