I'm not sure if this is intended or a glitch but something tells me it's just something that has been coded wrong.
On my Beastmaster, if my pet gets any links (aggression from other monsters) while fighting a monster, it will turn between however many monsters are currently attacking and not focus on the one I ask it to fight. It's incredibly annoying and a good way for a Beastmaster to die when there's no need to.
If I tell my pet to fight "X" monster it should not turn to "Y" monster and "Z" monster at it's own will because they're attacking it, it should continue to fight "X" monster.
Maybe this can be fixed at some point. Hopefully it's looked into.