I'm using Unity 2D as my desktop, and it works fine most of the time, but every now and then, I try to Alt-Tab to switch to a different application, and instead of switching to that window, the launcher bar opens, and the icon of the application I was trying to switch to starts jiggling.

If I try to switch to something else, that icon starts jiggling too. If I click on the icon with the mouse, then it stops jiggling and switches to the requested window.

What is this behavior? I haven't seen any mention of jiggling icons in Unity. I don't understand the original purpose of this, and I'd like a way to fix it so that I won't have to use the mouse.