I just figured I’d do my civic duty and let anyone else know what was causing my iPad to do this. I was replacing the homebutton on a lightly water damaged ipad Air 2, and when I went to turn it on it would flash red and keep looping trying to turn on. I removed the new homebutton that I installed and the ipad turned on and worked fine. So if you are putting on replacement parts they may have something to do with it!