Nerves..
Considering she dominated the ‘16 Olympics and everything since, i highly doubt its nerves.
There was an interview with Simone, before she withdrew, I believe. She said as she’s gotten older, she’s started to worry about risk, getting hurt, etc. ie “when I do this, this could happen”. So yes to “nerves” & not fearless any more. She also said that every day when she gets up to go the gym, there is pain - something is always hurting.
If I had to guess, I would say she’s going to retire from competitive gymnastics after this, based on her comments. I don’t think her heart is in it any more, either. There is a life beyond competitive gymnastics & I would think it’s time for her to find it.