Aggressive sail fin tang behavior

[Aurora1]

I recently had to return my sail fin tang to the local fish store after it started picking on our undulate trigger - a fish notorious for being the aggressor in most cases. But not this time, the trigger went completely submissive and just let the sail fin have its way. When the sail fin would approach, the trigger would just swim on its side and let it nip away without defending itself. It's weird, they've been tank mates for about 4 months now with no issues. I'm stumped, has anyone else experienced a similar change in behavior like this before?