Not based on gear at least. Surprised to see a dual weilding DK tanking these days. With a 155 hit rating he should be having some threat issues as well as being parry hasted out the ying yang.
The most likely reasons are simply friendship or experience raiding. I would suggest just asking if you could get a bit more tank time if you're feeling under used. Its a problem in 25mans as you often have more tanks than you need. At the very least, I would suggest dual-talenting to ret or holy so that you can fill a dps/heals spot when not tanking. What we tend to do in our guild is we'll bring junior 'mainspec' tanks as dps and let them continue to roll on tank gear to help them gear up. We'll also generally let them tank spider/plague for experience and eventually the harder stuff.
If you aren't higher than me in total damage done or total healing done, all comments will be /ignore'd