Upvoted because I never knew about this. As a user who prefers keyboard short-cuts, I usually press Windows key with D (alternatively, right-click on the task-bar and choose Show Desktop) if I want to minimise all windows.

Anyhow, I just tried shaking a window from my work laptop and discovered that this behaviour activates on the second change of the direction in rapid succession.

While I was experimenting, I also discovered that shaking the window again restores the other windows so you don't have to restore them individually if/when this happens accidentally.