[Glitch]

It was actually an intentional patch that was made on blue a long time ago, back in the days where charm was unusually reliable.
Basically, it seemed like to solve the issue of charm classes being too easy to play, they made all of the aggro generated by the pet transfer onto the owner on the break.

I don't remember that mechanic on live and uuuussuaaaalllyy they don't change things without archived sources, but go do some googling and pull something up if you'd like them to change it back.