I find the same people that like to open lots of tabs also prefer tabs that open in the background.
Why?
Because the browse breadth first. They'll be reading something and in the midst of it, see something they want to dig into, and open a new tab while continuing on the page they're currently on. This repeats a few times, and then once done with that content they go the next level down. Then lather-rinse-repeat. This causes a queue of pages to form (and BTW, for you programming newbies out there, this is a great way to visualize how recursion-less breadth-first tree traversal works).
I tend to be much more depth-first. When I open a tab, it's because I want to start exploring what's in that page. So I always have my browser focus new tabs (and screw you, Chrome, for only making that possible through an extension...). Then I'll pop back out and continue. In that model, you can just throw tabs away as you're done with them.