When going through a nether portal ( on realms ) you have a seemingly random chance that you could suffocate as the nether loads, you can't do anything you just have to wait as all items you are carying disappear from existinse others see in chat you died due to suffocation but you are stuck on the screen telling you that you are still loading nether then after a random amount of time you are sent back to the overworld at your spawnpoint.