Hi Erica
Samy does this to his baby bed at night but not to people (unless the person smells like a female dog then he might do it but not necessarily). What I think is, if you two were playing, he probably wanted to show dominance. Yorkies (females and males) show dominance by humping
When he does it, just tell him sharply NO and every time he does it say NO again and eventually he will know not to do it to people.
But this is really cute and funny hahaha they are sooo funny when they do it.. I watch Samy do it to his baby bed every night before wanting to sleep.. be pulls his baby bed into his big bed and does it hahahaha