I've found a couple posts on this, though they're unsanswered. I'm just really curious if this is expected or unexpected behaviour. I can't reproduce it, and when it does happen, I never think to check the log to see what other keys I might've inadvertently pressed (I'll do my best to remember for next time). So I have no idea what might be happening. I thought I remembered coming across a post that said you could use ⌘-Tab to activate the application switcher and then hit another key that would allow you to release ⌘ entirely and have the switcher remain on the screen, allowing you to navigate it with the arrow keys and selecting with return—but I cannot find that post now.
[edit] As luck would have it, it just happened randomly. I checked the log, but nothing is different from usual.