VERY ( OFF TOPIC ) my dogs can't stop humping !

So I have two female dogs not related around the same age ( 1 year ) recently I noticed they hump each other which is so crazy it doesn't make any sense to me I mean one is spayed and the other isn't could that be it ? I don't know but they hump each other at least once a day if not more and their both female dogs I just can't understand does anyone know what this could mean ? PLEASE HELP