If you mean physically, I assume it occurs through the same mechanism as this kind of thing:
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When you are deeply unconscious (via alcohol or general anesthetic) you won’t wake up a little to adjust your position when something starts hurting, so in the wrong position you can injure yourself and fuck up nerves just with your own body weight. In her case it looks like the nerves were damaged permanently.
If you mean “how could surgeons and their team let it happen” I have no clue. This is not the first trans person I’ve seen here talking about nerve damage after surgery. I think SRS is unusually long, but plenty of people get long surgeries and don’t come out with unnecessary nerve damage. Possibly because the doctors doing this stuff don’t give a shit and are just in it for the troon cash bonanza.