Starcraft has waypoints too (also with Shift, I assume this was a UI convention just taken from Windows)

But in this particular case, what's happening is an element of pathfinding where if something goes wrong and the unit ends up in an invalid state/location, it's bumped in a random direction to get it out. Here, I think the SCV and Dragoon end up accidentally occupying the same space, which causes the Dragoon to get bumped downwards - except that bumps it into the water, which is also unpathable terrain for a ground unit, and so it keeps getting bumped down until it eventually occupies a valid spot of land.