Livi has done this to me and my husband a few times. She has pottied on our feet and has also jumped into our laps while we were just relaxing on the couch and peed in our laps (talk about shocking!!)
The times that she did this to our feet, she was around the same age as your puppy and she was still struggling with potty training. In fact, one of those times I even remember telling my husband that I had seen her sitting by the backdoor and then she came and pottied on his foot. With Livi, I truly feel like it was her way of telling us that she needed to go out. Now with the times that she jumped into our laps and did this, she was big and pregnant so I think it was more to do with this.
I wish I were more help...just wanted you to know that you are not the only one that this has happened to.
I would just suggest keeping a REALLY close on your Yorkie's "potty cues".
Good luck!
Tammy