[tranith]

This happened because you castes firefield, and part of your firefield hit him when you finished casting.
You will only turn crim (start attacking them) if your firefield lands on them. You don't go crim if they run through your field, though.
You just have to be careful where you are casting fields/walls. If you accidently land one on somebody (lots of people will try to get you to land a wall/field on them) then you will go crim.
Hope that helps

Makes sense