I just finished reading "This House of Sky" by Ivan Doig, and he talks about sheep running off a cliff when panicked during a storm. I think what happens is that when the flock is moving quickly in a tight group, the leaders may try to stop at the edge of a cliff and get pushed off by the mass of the group behind them.
Indians used to hunt buffalo in a similar way, driving them off of cliffs. There are huge prehistoric kill sites where archeologists recovered hundreds (more?) of bodies.