Schven, your right, the shoe of the pantograph only pushes up on the bottom of the catenary. What C-H was asking is why a derailment brings the "cat" down. The cat comes down when the shoe, that is supposed to be under the wire, hooks over the top of the wire due to the locomotive wandering around places it's not supposed to be, eg, the tie's (or sleepers if your in England) instead of the rail.
TW
