Why does my one mini dachshund who was......?

spayed at the age of about 81/2 months always mount and try to mate with (like a male dog) stuffed animals. My other female dachshund who was spayed at age 4 months doesn't do that. She never went into heat as I waited quite a long time to get her spayed. I don't understand why she does it. I don't reprimand her and am not really bothered by it. I am just curious as to why she does it and if it is common or not.