Bag Glitch. It's a well-known visual bug by the community. related to loading causes your bag to fall off of your back for a player. That player can then pick up your bag without you even having to take it off first. I can't exactly give reproduction steps for it, but this bug is very known and to be honest I don't even know fully why it happens. My guess is something related to loading doesn't load the bag being on someone's back, so it just doesn't. A fix (if that's the case) could be to prioritize loading bags on people's backs over anything else.