Some dogs are Jack Rabbits, others have a slow more fluid motion and some dogs smile when they are humping. If you search Youtube you will find an endless supply of dog humping videos. For humans humping is private and it is something consensual. That’s probably why we think it's so hilarious how dogs in public will hump just about anything, anyhow with no shame.

Why do female dogs hump?

Sometimes it's sexual (if a Bitch has not been spayed) or dominant behavior but mainly humping is a result of a heightened arousal state, which may be a stress response or pure bliss. It could be that your dog was excited when you had guests over so they humped or maybe they became stressed out at the Vets

Spaying might not help either if its related to stress or arousal (excitement)

Why do male dogs hump?

Male dogs hump for similar reasons. They might be dominant or horny (if not neutered) or it can be a learned behavior from when they were unneutered. If they are neutered it could be arousal (stress or excitement), it just feels good or maybe some of that lingering testosterone that makes them want to show who the boss is.

If your male dog is unneutered and likes to hump you should keep them away from other dominant males and dog parks.

Humping is pretty harmless as long as the object is soft and humping is not overdone. There are dogs that overdo it and get rashes.

Also NEVER allow your dog to hump another dog because it could turn into a dogfight or injure an older dog with bad hips or arthritis

If you want to stop humping you can train your dog with the command word, “Off” before it becomes a habit and its probably best to never laugh or scream or show excitement as this will get your dog to recognize humping is fun