User blog comment:Miskos3/Fairy Tail Chapter 505: Trump Card, Review/@comment-97.104.204.83-20161010124046/@comment-1776824-20161010165749

Mavis was cursed by casting Law because she knew that it was an incomplete magic that could very well have backfired and taken the lives of everyone in Magnolia, yet she cast it anyway not knowing what the result would be. It was that action of making light of the weight of the lives of others that led to her obtaining the Curse of Contradiction. It makes perfect sense, imo.