Not many people can make the fine distinction between a yelp and a panicked yell. It's one thing when Greg thinks he's the one in danger, another entirely when someone else is.

It's also hard to make the distinction between a thoughtlessly stupid move and a planned stupid move. Rushing over to grab at the killer cloaks wrapping around Sans' bones could qualify for either. Greg has no weaponry, nor a strong enough grip to pull them off. But, he is a much meatier meal for them to latch onto.