I think this will apply:
From the Compendium:
Q. Can Dugtrio use "Sinkhole" if the opponent uses Switch or Warp Point instead of retreating? Or what happens if I attack with Dugtrio's "Push Down" to send the Defending Pokemon to the Bench?
A. Only the game action of Retreat triggers the Body's effect. Switch and other effects that move the Active Pokemon to the bench do not trigger Sinkhole. (Feb 12, 2009 PUI Rules Team; Feb 26, 2009 PUI Rules Team)