You can also see the problem the other way round :
you don't assume a defensive stance because you know what the opponent will do, you assume it because one of your companion is having trouble, or is wounded, and would risk death if hurt one more time.
The decision comes from your will to help someone, not to count your actions.
If, in the end, the monster chooses another target then the wounded companion is safe.
The LM could specify if the monster seems to target someone in particular if it is the case, or if this monster just hits randomly, not to surprise the players.