my friends’s dog keeps on acting AGGRESSIVE when we’re outside 😭

okay for context, my friends’s dog’s is kramer, a white labradoodle, and EVERY time me and my friends go out to the backyard and he’s out there, he’s always barking and running at us, and everytime we TRY to go further out to the backyard, he starts chasing us, barking aggressively, and trying to bite us (me and my friend got bit like several times but it wasn’t that bad), but for some reason, when he’s inside, he’s calm, and this makes me wonder, what causes this behavior? i’m not an expert on dogs but i’m just wondering why does he get so aggressive when we’re in the backyard with him? cuz this is getting out of hand