I think this is actually a bug, seeing that you both ended with the same score and no one really played.
You see, people higher on the leaderboard lose more ELO, and gain less. Here's how it works: If you ever are in a ranked map and leave, in the end it will display your name with the amount of deaths you got along with the fact that it resets your kills to 0. Since that person got no deaths nor kills, nothing changed. But your score was the same as theirs, and you are higher on the leaderboard. Because you had the same scoring (your score was in the end the same as theirs since they left) you were the one to lose ELO and they gained this. That's why that happened. It's how the system works, and it needs to be fixed.
Quite sure the person who did it can get banned for abusing a bug cause this happened a lot last reset to me and the people who did it got banned so I don't know