"Log out (username)" logs out all users

When I click on "Log out User A" in the Apple menu, then click the Log Out button, the current user is logged out. After I get back to the user selection page, I can pick another logged-in user (call them User B), but almost as soon as I see their desktop they start logging out as well. By the time I return to the user selection page, all users including User C and User D are also logged out.

I saw this a few times when using they keyboard shortcut (CMD-SHIFT-Q) for logout, where I'd quickly switch after logging out a user and see the "log out" popup prompt for an unrelated user, but I figured that somehow the keyboard input was being relayed to a background user's account (which would also be crazy, I grant you, but less crazy than this).

I think it started happening when I upgraded to 10.10, but I wouldn't swear to it. It's happening almost every time I log anybody out now, and I'd really like to make it stop. Help?