This is a simple one, I just remember the "rake in or rake out" analogy. If you leave the rake in the far side of the bunker, and the bunker slopes forward - then if the ball catches on the rake, you can't place the ball without it rolling towards the hole. You'd have to take the ball out of the hazard / behind the bunker and take a penalty - hence, that is why the bunker should be left out of the bunker.
Similarly, for TP. If the TP catches on the rake, and you remove the rake and try to place the TP. If the TP loose end comes from the back to the bottom, then the TP rolls forward and unrolls. A terrible mess! But, if you place the TP with the loose end coming to the bottom, then the TP rolls forward, and the TP doesn't unroll! Much better!
For that reason alone...under the bottom!