There were two bugs in my game.
The first was when the 3/3 battlepet spawned creep on a space that already had creep. First, my 1/1 Battlepet hit it, then a Makantor hit it, and the creep spawned on the space both times. You can see the dust flying up in both instances in the bottom left under the Young Silithar.
The second bug was when the enemy general hit one of my battlepets with forcefield. After the general hit the minion and it teleported away, the enemy's Abyssal Juggernaut hit it (even though it was not adjacent).