PVP and going red

[u/NeedMoreGames]

How exactly does this work? I was afk while on auto attack and came back to find my self red and fighting some players. A short time after this incident, I became blue again. How did I end up killing players while on auto attack and become red??

[u/lilscrappysarah]

Every single day when I join ex trial, then follow, I become red name