Discord has long since been forgiven for his betrayal and conspiracy with Tirek. The six mares and dragon have given him a second chance, one he knows he surely didn't deserve and gradually they have all been able to become a bit closer in the circle of friendship. While everyone has put his treachery in the past, Discord himself never forgets. There are times Discord recalls how Tirek betrayed him, there are times where he remembers how he hurt Fluttershy and all the others, because, for one brief foolish moment he thought Tirek was his friend.
And these moments infuriate him to no end. So on one fateful day the Lord of Chaos makes one heck of a decision.
He pays Tirek, a visit.