I noticed the same thing some time ago with the Bigby spells. For some reason, the AI commands them to move near the person who cast the spell. It should simply immobilize them where they are.
The AI scripting is very complex. It's on my agenda to redo it, but I'm still using mostly Bioware's scripting to operate the monsters in combat. There is quite a bit of coding involved, so much so that it amazes me that the game works as well as it does.
Think about it. At the end of every round, the monsters have to decide who to fight, analyze the situation, and determine the best course of action. If they are casters, they have to determine which spell (out of many possibly memorized) to cast. They also have to check their inventories for items that might be used.
Now consider that these scripts are run for every single active monster every round...
BTW, Bioware does have a neat trick for monsters who are in areas without PCs. They basically do nothing, so if you leave an area where monsters are fighting monsters, the fight stops until a PC re-enters the area.
As for the spell casting PvP settings, about 2/3 of the spells are changed over to the new system. They only hurt hostile creatures, and you must "dislike" a player to even use the spells in PvP. I haven't extensively tested the coding in all circumstances, like where one of your henchmen casts a spell in an area including NPCs, monsters, other henchmen, etc. So please let me know if you see anything weird going on.