Everytime I cast her 'q' on minions or jungle monsters, it would do initial damage, they would have about 10-20 health left on them. Then when they dropped from the bubbles, they would take additional damage and die. I don't see this anywhere on the tool tips of her abilities so I'm assuming that it's a bug. Two damage phases, initial hit with the skill, and then again when they hit the ground.
Location didn't make a difference to it. It wasn't anything relating to how long the game had gone on. It was happening only when the health on the minion or monster was between 10-20 points.