When standing on or walking into rigid entities (boats, shulkers, etc.), the server will occasionally not detect a collision between the player and said entity and will move the server-side position of the player in the direction of movement despite the client not registering the movement at all. After what looks like 10 ticks, the server will correct the position and has a high likelihood of doing it again in repetition.

To replicate:

Setup a repeating command block to spawn a particle at your feet every tick:

/execute at @a run particle minecraft:large_explosion ~ ~ ~

Spawn a shulker on a wall above some air and stand on it. Walk around on it, jump on it, sneak to the edge, etc. Eventually the server will trip up and send the particles (your server-side position) through the shulker and onto the ground for about half a second.

This will not always happen, but tends to happen after an event that causes server strain such as running the /reload command. It may take some playing around with it to replicate it.