Humping or mouthing/certain biting behaviour is not always a sign of dominance or even in the case of humping a for sure indication of mating behaviour either. Humping or mouthing/certain biting actions can be done by dogs when they are overly excited(and this does not have to have anything to do with sexual excitement) and is often done by puppies to their blankets or toys to build up their self-confidence. There is even evidence out there that suggests that humping and/or mouthing/certain biting behaviour is being used simply as a sign of affection. (A great sign obviously!)
There are also many dominant dogs out there that will allow a submissive dog to hump them or mouth/do certain biting movements to them, the dominant dog; yet would never allow another dog to do so.
Dogs can also hump due to anal gland issues or having an UTI(the dog doing the humping or the dog being humped).
Some dogs simply hump/mount as part of their play repertoire or, if both are female, one of the dogs could be coming into heat(the one doing the humping or the dog being humped).
Author: Gwendilin Boers, 2014