Rashi: They are two harsh matters next to each other. Fear causes him to flee, and the pit is prepared for him to fall in it.

Malbim: To hunt Chayos, they dig a ditch and a big pit in which the animal will fall when it goes. On one side of the ditch they place a snare, that if it tries to leave, it is trapped in it. In order to make it fall into the ditch, they scare it with dogs. Due to the fear it falls in the ditch, and through the ditch it is caught in the trap. So will happen to Mo'av!