Quote Originally Posted by Wolfsbane706 View Post
If there's a large level disparity between yourself and the beastie you're attacking, it'll have a partner when it retaliates. This includes neutral monsters.
It's also affected by whether you've got your companion chocobo out - withdrawing it stops the second enemy from attacking.

It seems like attacking an enemy applies a certain 'amount' of enmity relative to your power level, and if it's over 100% of what they can take, it overflows into the next closest enemy.