Trolls has an invisible item making sure they don't die - just fall down - when dealt lethal damage. Further evolutions, like troll's regeneration, are handled by troll's script. Perhaps Spells50 tweaked Sunfire (spell you use) in such a way that it now bypasses said item's protection, killing trolls outright thus freakng their scripts with result you described. Had to check troll's script to be sure though