My cat goes nuts eating plants

My cat, Jasper, loses his mind when he goes outside. He starts eating vegetation like a starving herbivore and gets really angry and defiant if you bother him. It’s hilarious! Inside the house, he is the most even tempered, cool cat😎 . He won’t even hiss at other cats. When he’s outside he goes nuts!

I think it could be because he was homeless as a kitten. He must regress into some sort of survival mode. He becomes hostile and eats like there is no tomorrow. He also eats insects. He can catch a fly in his paws. Flies that get into the house don’t last long with Jazz.