Taking the upslide for granted (which I could only explain if Mickey had some big magnets in his pockets -- and even then, the movement wouldn't be linear, but parabolic), this can make sense, depending on the slide's architecture.
I've drawn a picture that highlights one possible architecture, omitting the curliness of the original slide. However, in order not to infringe on Mickey's precious copyright, I took the freedom to put Tuxes on the slide.
In the picture, the blue tux is sitting in front of the green tux, that is closer to the end of the slide (like Mickey in your video). If the upslide magically slides them upwards with an initial velocity of v0, they will end up in a position in the middle, the blue tux being closer to the ground. I'm assuming that the slide is high enough so that all initial kinetic energy of the tuxes can be transformed into potential energy, meaning that they come to a full stop for an infinitesimal amount of time (v = 0).
At that point normal physics kick in and gravity pulls them down. Choosing at random (if the slide is built in a safe fashion, otherwise ... well let's say the occupants couldn't have future offspring otherwise) or with a bias the right side of the slide to descend. Note that this will restore the original kinetic energy and they will get off the slide on the same velocity as initially accelerated to (if you disregard friction) as well as it restores who was sitting in front (i.e. the blue tux).
Note: This is not canonical, but I don't think Disney would bother with physically correct entertainment apparatuses.