tab function sometimes changes

Generally, my tab button works in the "expected" manner - i.e., if I'm trying to access my yahoo mail or something, I'll type in my user name, hit tab, then type in my password.
Every few months, though, there'll be a time when I hit the tab button and it takes me to my last open window instead. Can't figure out any way to undo this action of its, short of rebooting altogether.
Am I actually DOING something to cause the tab button to perform a different function, or is this just a quirk on my computer? Is there a way to get the tab to go back to its original function without having to reboot?

 

Well I'll be .... Yep, that did the trick!
Many thanks, Steve!