[Gameplay] Double Kill on Tristana?

I was playing a ranked game when a fight breaks out between the bot laners. The enemy team was winning and was about to land a double kill with one last kill. However, when the enemy Tristana killed our Malzahar, the kill registered twice, giving Tristana a triple kill instead of a double. I am not sure how this happened, and I doubt I could recreate this again. Rito?