Slime Blocks Throwing At Angles

Me and my friend noticed that quite often, that slime blocks on an activated piston would throw you at weird angles for no reason, as shown in this video https://www.youtube.com/watch?v=358J_r08ih0&feature=youtu.be. I pressed no keys during the test, and 2 times out of 5 I would not be able to ride a functional elevator. We though this was the problem with the elevator we built, but as you clearly see it is with piston slime blocks in general. It would be most appreciated if you would confirm this as soon as possible. Thanks!