Is he just mounting your Mastiff or is he that way with other dogs he meets? Knowing his health history, I would take a trip to the vet to rule out problems where that is concerned, especially since this seems like a sudden change. Also, sometimes dogs act out on occasion, that's why training is an ongoing thing. For now, I would make sure to close the garbage can or keep it out of his reach and not leave anything yummy on the counter when someone isn't there to supervise. You also might want to consult a behaviorist if the behavior continues and there is nothing medically wrong, especially if you don't know what could possibly be the stressor. Another thing, is he getting enough exercise during the day? I know you said he has hip issues but if he is hyper, maybe he could benefit from some long walks, swimming, etc.