A crushing force to the side or rear could disrupt a units ability to fight forwards, resulting in people getting pushed back from both directions.
More specifically, though, it has to do with how units of infantry fought together, shield wall to shield wall in what were sometimes described as giant shoving matches. That sort of thing still works crossfire is, if anything, even more devastating with modern weaponry. At its simplest, it's just a case of surrounding the enemy, which is virtually always bad.
