Pet aggro, is it bugged?
If you engage a mob and then run out of combat, and the mob comes back in to pet range, it may need to be recalled so it doesn’t attack them (this often happens in dungeons, CoF1 with the flame acolytes is the most common place.)
It can be a bit tricky to tell if you’ve engaged the mob. If you run by and don’t attack, but are attacked and missed or your pet takes damage, your pet will sense the aggro and view them as enemies, and will again need to be recalled before he becomes able to be aggrod.
Any of that sound like your situation?
You can just hit F3 and it will call your pet back. However, I think that you are probably using some skill (any weapon skill, for example warhorn 5 for swiftness) while the enemy is selected on your screen. That will make the pet engage that enemy.