When a dog is in heat, her body is preparing for pregnancy. This involves the release of eggs from the ovaries, and the thickening of the uterine lining in preparation for a possible pregnancy. These changes can cause your dog to behave differently, and one of these behaviors is humping.
Humping is often thought of as a sexual behavior, but it's actually quite common in both male and female dogs and isn't always sexual in nature. For example, puppies will often hump each other during play and socialization sessions. And some dogs will hump objects or people when they're feeling anxious or stressed.
In female dogs, humping usually occurs right before they go into heat. It's thought to be caused by an increase in hormones like estrogen and progesterone that stimulate the sex drive. Humping can also be a way for your dog to relieve tension or frustration if she's not able to mate during her heat cycle.
If your dog starts humping more frequently or aggressively during her heat cycle, it's best to provide her with lots of toys and attention to keep her occupied and distracted. You may also want to consider having her spayed after she comes out of heat, which will help prevent any unwanted pregnancies and reduce her overall desire to mate.