Occasionally when clicking on an open application in the dock (e.g. Finder.app) another window will open with it, the best example I can give is that when I am in Photoshop and need to get something from Finder I click on Finder and Firefox (which was already open) shows it window. This is very inconvenient and I cannot find a correlation between when this happens and when it doesn't.
As it turns out this can also happen when using
Cmd + Tab.