Maybe they just gotta go? And it has no real meaning behind it? I don't know.. maybe the minor stress of moving a male out of his tub, into a strange new tub, brings it out?
If mine urate/defecate when paired, I just leave it be for a day or so. If, say, they lock up on the fist day, and day 2 they are seperated, I will clean then; I don't want mold forming. But I don't want to interrupt them either when they're locked, so I'll leave well enough alone until the deed is done.
Anyone notice tail-wagging in their females? My one girl does it incessantly when I put the male in! She did last year, and it led to a successful breeding.