If the player has a motion tag but is stationary (such as being pressed against a wall), switching to spectator will carry over the motion tag, causing movement, instead of respecting the fact that the player is stationary.

An easy way to duplicate it:

1. Dig a 2-block hole
2. Spawn a villager inside
3. Go inside the hole
4. Switch to spectator mode

At step 3, the player will be forced into one of the corners as a result of constantly colliding with the hitbox of the villager. This constant pressure causes the player to have a motion tag while not moving. When the player switches to spectator mode, there is no longer any wall to collide with, so the player's motion tag causes them to go from stationary to moving.

This seems like something that could be a specific case of something more broad (motion tags not being handled well? Mob hitboxes continuing to apply the motion tag even after they should've stopped?), but I can't think of much, let alone test for them.