When passing through a nether portal, sometimes (i only experienced this on a server) your minecraft client freezes and when you log back on the server, you are at the coordinates of the portal but in the other dimension. That happens because the server doesn't have enough time to divide or multiply your coordinates by 8 before you've been kicked. On this coordinates, you can eventually see some leads. That's because everytime you pass an animal with a lead through this portal, the lead breaks, glitches and goes to this location. As a "proof" I can link a video of Mumbo Jumbo, he experienced this bug at 2 minutes : https://youtu.be/ljZmeRSPzm0