I've got animal friend perk rank 1 and have noticed that the animals have started attacking
my enemies which I didn't think they were supposed to do until rank 2.

Is there actually a difference in behavior in rank 2 or is this glitch?

Hmm, i thought that was the 2nd rank. Odd. Sorry, not much help from my post.

There isn't much of a difference. At level 1 the animals ignore you, so aggresive animals will ignore you and attack other things providing that other target is in their aggro range. At level 2, animals will come to your aid if you are attacked. Unfortunately for me, the number of animals seemed to go down if I took level 2. Level 1 was worth the slot; level 2 not.