Clear the anal glands which can get gummed up.
You might also just check to see if some poo has gotten stuck. It sometimes happens with my dog and it is the only time he seems to drag his rear on the ground. Also, check your dog for insect bites - my dog is allergic to the things he gets bitten by when rolling in the grass, and he ends up rubbing his body on the carpet in an attempt to itch himself.
maybe he has worms or other around his butt, cause our dog did that (drag her bum) and we found she had worms.
I just took my yorkie to puppie classes and they was saying if you feed your dog alot of grain and corn this will cause the butt dragging
I had a yorkie for 16 years...get him fixed...after that no more dragging butts and humping my kids' stuffed animals...trust me
Yorkies get their anal gland clogged. Ask your vet how to get it out. Usually you could squish it out but you seriously have to.