It's a dominant/social structure behavior. Your dog is showing the toy who is the boss. You could take it away, that may fix the issue.

I have three females, big, bigger and biggest. With three dogs in the house, there is a clear pack mentality going, always visible. I have noticed not as much humping by my alpha, perhaps she is more comfortable with things.

Bigger is my alpha, she humps "big", never biggest. When a smaller dog visits or even a human infant, "big", the bottom dog in the pack, humps them.

It is embarrassing, I try to keep in from happening, especially when guest's are around, but not always successful.

SG

SG is correct, just put her toy up when company is there if you dont like the action. You might add a bit more training, that will help your pup and how she views you

I have this problem with my two girls to.It is embarrasing.My youngest humps the alpha female.Occasionaly the alpha humps the younger one but not often.They each have certain areas they do this in,for the younger one its when we load them up for the jeep ride around the block,I think she uses that time as the alpha female is distracted,she loves her ride.We just make them stop when they start with the humping.Its mostly embarrasing because most people think its a sexual thing not a domaniance issue.