User blog comment:FairyRaven/Fairy Tail Chapter 362: Natsu vs. Jackal, Review and Commentary/@comment-4829046-20131129122515/@comment-4829046-20131129123326

I don't think its anger that triggers it, more like him finally deciding that he needed his actual form to defeat Natsu, similar to how Tempester transformed when he realized that Laxus was going to be a bit of a challenge.

And technically he became a "Werejackal" =P Which is why I laugh at the people that called him Jaguar.