OK, this just started happening recently. It's not a big deal but I'm curious as all get out to know what might be causing it.
Here's a simple scenario describing my win 8.1 x64 system...
I run 1 win explorer windows listing say D drive
Then I open another explorer window to the C drive

Now on the taskbar u can see that 2 windows are active so I hover the mouse over it and the 2 preview windows pop up.
Now let's say the C drive is on top right now and I want the D drive to come to the front so I click on that preview window but it still stays on the c drive. BUT the SECOND time I click on the D drive in the preview thumbnail it will come to the front.
Also when I try to bring an explorer window to the front when it's behind another program it won't do it. I have to click on the other program to minimize it. u can tell the explorer icon becomes the active window and the other is dimmed but explorer doesn't come to the front.
It's always when it's interaction with a win explorer window. Between 2 other programs everything works as it should.

Like I said, no big deal but how the heck did this start happening?