Other mistake: It isn't known if Rory was diagnosed with ASD, but some of his behaviors and the two classmates in school making a reference to his "Asperger's" indicate or suggest this is the case. While Rory may have been "faking" some mannerisms in hope that the two bullies would leave him alone, his (hyper) sensitivity to noise (sound) was displayed after school when a dog was barking at Rory. However, Rory did not put his hands over his ears in the ball field despite the loud gunshots close to his ears. (00:51:54)
Other mistake: At the baseball field after Quinn (Rory's dad) slid through the green slime, he only had green slime under his neck on the front of his shirt - none on his pants or back of shirt where expected. (00:52:52 - 00:53:37)
Answer: The Predator had landed in the middle of a group of hostile Earth soldiers, who it judged would likely have just shot it immediately. It would have defended itself. And there's also the fact that it could just be for fun because, hey, old habits die hard.