Janet, it could be caused by a peristomal hernia, a hernia of the muscle around the stoma...it would cause a loosening of the tissue that holds the stoma in place and would subsequently allow it to release or get popped out a bit.
If, and I am not saying that it is, it is the case then a simple patch fix can work miracles.
I had a mesh stoma repair done that was 100xs more effective than the open surgery that I had had done.
I hope that it is not the case but it is one of the possibilities.
Sharon