I think this is more like good information to keep in mind in case it ever happens rather than something to worry overmuch about. It happens, I wouldn't say it happens a lot. Umbilical hernias can happen when the umbilicus doesn't close up after hatch. Some hernia's can be caused by an injury.Nothing better than the Vet! Guesswork & a chicken community can only resolve so much.
I wonder how old is your girl? Is this something we need to watch for at a certain age? Being a newbie, I'm getting so much intel from this site. Thanks!
In 11 years I've never had a hernia in my birds. Doesn't mean it couldn't ever happen, but it's not a huge worry as a daily thing.
More info:
https://www.mvtimes.com/2012/06/13/differentiating-between-tumor-egg-hernia-11064/