[Scott's Trick][1] is called a "trick" because it is not actually necessary for the completion of the proof in which it is involved; however, without the trick the proof is massively more tedious.  Although the other tricks may not have a widely-agreed-upon-reason for being a trick, I suspect that they may be called such for similar reasons.

  [1]: http://en.wikipedia.org/wiki/Scott%27s_trick