Basically /petattack will aquire a random hostile target if you don't have one already due to the lag issues mentioned above. Try setting up a macro like this:

/assist focus (or whoever you normally assist)
/stopmacro [noharm]
/petattack

This makes sure you will only execute the /petattack command when you have aquired the hostile target from the assist. You might need to mash the buttons a few times to complete the macro, but thats better than random pulls wiping the group IMO.

Cheers,
Stealthy