My almost two year old neutered Wheaten Terrier has started biting my toes. He doesn't bit hard. He is usually an extremely affectionate friendly dog and although he has a stubborn streak he is normally very well behaved and obeys sit, stay, down and off commands well.
It almost seems as if this happens when he is frustrated with me. If I am not doing something he wants. Today he was having a play date at my friends home (he's been there many times before) but suddenly ran across the room and bit at her daughters (20 years old) feet as she walked by.
What can I do to stop this behaviour.

Has anyone pushed him with his/her foot, kicked at him, anything like that?

Is he allowed to play with people's feet, socks? Has he been allowed in the past?