At least once or twice a day, someone posts a thread to the official hunter forums that says something like "every time I fight something, once it's dead my fox/bear runs around and kills all the nearby critters. Why is this?"
There's a lot of speculation in every thread. My favorite guess has been "you know how mobs will sometimes run over and kill a critter in the world, to look like they're hunting? That little bit of code is somehow still active in your pet." While this is creative and fun to imagine ("aw, my pet is hunting!"), this is not what's happening.
Bears have Demoralizing Roar and foxes have Tailspin. Both of these abilities create an AoE, non-damaging debuff that puts them in combat with everything nearby but doesn't do any damage. This means that any critters in the vicinty are getting this debuff without dying, so that as soon as your main target is dead, your bear or fox goes "but I'm still in combat with these critters! Better kill 'em!"
Also, if you've killed one of the Cataclysm rare spawn beasts, you may have noticed that they drop an interesting vendor gray. I hope your 25 gold is forever tainted by your guilt, murderer!