Riding Hobbyhorses

I have always wondered why this equestrian metaphor has been applied to someone who waxes eloquent on his pet subject, regardless of whether it is apt in the circumstances. It has pleased me no end, nonetheless, to have this so applied since I am a past master at this and the metaphor sort of makes me feel that I have added horse-riding to my immense repertoire of skills. There is one hitch, however. It is impossible for someone to appear elegant while riding his hobbyhorse.
Even the geniuses...