This was a case of Sparky deciding the game was lost early - 10-3 in the 4th. So he put in the AAA mopup, who actually did ok for two innings but then let it get way out of hand.
I have seen enough weird comebacks in SIM baseball and have seen a few in RL baseball. So I usually put my manager settings on defensive replacements if behind by 7 runs in the 8th or 7 ahead in the 8th. Not before. I usually avoid having a designated mopup unless there is a pitcher that is really awful I need to have on the roster because Sparky will use him if my team is well behind.
Long B is good enough I think, usually a below par but not disastrous pitcher for when you are behind. But usually you do see these kind of blowouts when one team is tanking to rest its starters, not when the real starting lineup and pitching staff was in.