You see, silvertips can smell your fear, just like any other scary-ass predator. But unlike most predators, they can smell your fear over distances measured in double digits. In miles. These bears can smell a carcass from 18 miles away. When a silvertip enters a clearing, you're screwed even if you left 48 hours before. And if you're still there, all that sweating, pissing yourself and crapping your pants while you ran away will be laying down a neon-colored trail for the bear to follow until you get tired.
Part of the reason for this is because, despite what some scientists would have you believe, predators are scavengers as well, and the ability to smell a dead body close to 20 miles away goes a long way toward helping get a free meal. Since bears scare the shit out of every other living thing (except maybe wolverines), being able to smell someone else's kill is like being a 250-pound bully in eighth grade; no one is going to fuck with you when you walk up to him and take his food. Not even entire packs of wolves.
Being a gigantic bear is nature's way of saying, "Fuck off!"