Does Slithering Behind You cause the encounter deck's discard pile to be shuffled into it?
No, it does not. Slithering Behind You interacts with Hunting Horror. If that is not in play yet, it makes sure that that happens - by searching in the available cards, in the discarded cards and in the void.

After searching the Encounter deck, you have seen the order of cards. So that needs to be randomised again. This does nothing with the discard pile.