I don't know what causes it, but I left my base on my donkey (with saddlebags) with one lead and one fence post. I stopped to tame a horse to check its speed, and noticed that I had three leads. I had stopped a few times previous to finding the horse, but I did not notice at what point the leads were duplicated. I left the horse tied to a post with one of the leads, so I then had 2 remaining. I stopped again and tied my donkey to a post with a lead, and when I remounted, I had six leads! I don't know how this is happening, but its out of control lol

Edit to Add: Leads are also disappearing. I had llamas attached to leads, but noticed that at some point the lead disappeared and two of my three llamas were gone