I think it's a dominance thing. They're establishing and enforcing a heirarchy. I don't think it's really 'humping', just one rat getting on top of the other as if to say, "I'm higher than you...literally". It's not an uncommon behavior in any animal, really. Any animal that has a social structure for survival will form a heirarchy and that heirarchy is usually 'shown off' by the dominant one taking a higher stance over the submissive one. Even humans do it. Why do you think it feels like someone is declaring themselves superior when they walk around with their nose in the air? It's an instinctive reaction to a subconsciously dominant stance. Most humans don't even realize they do it.