In CS2, when a player is sliding/surfing against some geometry, it's possible that they lose all their momentum/velocity. This is usually called wallbug/rampbug. While the name was inherited from Source 1 games (such as CS:GO, CS:S, TF2,...), rampbugs in CS2 behave much differently from its predecessor. In contrast to source 1 games, it's also possible have the velocity redirected to another direction instead of losing all momentum.
This bug doesn't seem to be common in all source 2 games (HL:A for instance, does not have this bug), and was orignially not present in the very early versions of CS2 Limited Test. Rampbugs become more and more frequent over time, with the Call to Arms update effectively doubling the frequency of these bugs, which is a significant problem for custom gamemodes heavily depending on geometry collision (eg. surf).
Keep in mind that while the player collision hitbox is a box, the images shown below will represent the player as a dot instead for simplicity.