some years ago we had a similar problem. the finding then was, it happened because we had set some volumes offline for some time. seems like the compliance time also stops.... seems logical as SnaopLock need to make some guarantees, right?
if this is still the case, you nowadays need to be careful also with "vol destroys"... because, if I remember it correct, ONTAP keeps those destroyed volumes in a queue for some time... maybe that "queue time" is the hour in your case / settings?! so all together, that would explain it...