When a group of people are all moving in one direction, like into a bar or concert, the one or two people who are moving in the opposite direction of the crowd bumping into everyone and getting in the way, like a salmon swimming upstream.
example 1:
"this guy just bumped into me and made me spill my beer, what a fucking crowd salmon"
example 2:
person 1 "whats this guys problem, hes getting in everyones way"
person 2 "hes a god damn crowd salmon, he loves it"