Nostalrius Begins - Quality wow vanilla realm (1.12)

Sometimes when I use /reload, my pet will become completely unresponsive and do nothing but follow me around.

My pet action bar disappears so I can't give it any commands, either. Even using a macro to command it to attack, does nothing.

The only way to fix it seems to be dismissing + calling pet, or relogging. What's going on?

Yeah, that's a very annoying bug, especially on Alterac Valley.
There's nothing you can do about it, dismiss pet + call pet is the only option.