buckling would be wrinkling of the tubing. If you mashed the fork backwards suddenly, the impact force could cause the bottom of the head tube to move backwards and/or upwards. That would cause the top tube and the seat tube of the bike to get a little squashed. It is usually not a very big deal, but if it is heavily bent, it could cause some bike handling problems, and in extreme cases it could be dangerous.

another probably outcome of a heavy frontal crash is that the fork is destroyed, and acts as a fuse to absorb all of the impact energy, leaving the frame pretty much okeydokey. The more expensive the bike, the more likely that a frontal impact will damage the frame. The cheaper the bike, the more likely that the fork will fail.

Expensive bikes have light tubing and stiff forks...cheaper bikes, not so much.