I'm not sure if we ever got a conclusive answer for that but I do remember one theory saying that, since the Garuda in the Ixal legends became bloodthirsty after eating the flesh of the Lord of Snakes (or...something like that?), in their minds, Garuda is insane because of that and the Ixali don't realize they COULD make more noble and calmer, they're simply trying to summon their Goddess.

Or you could choose the other option, that Garuda is like that because she's aspected to Wind, so she's flighty and capricious. After all, she COULD be as calm as a summer breeze or as wild as a tornado, and the Ixal summoned her because they wanted her to destroy the Garleans and Gridanians, so she ended with a destructive nature. Or...something.