I think it started as simple curiosity based around her illness/surgery being made so secretive, as without this initial "hmm that's odd, why is it secret" niggle at the brains of people who genuinely like Kate, this current photo faff would be virtually none existent.
Yet due to both the initial secrecy, and now a doctored photo, people are thinking (correctly/incorrectly) that they've been lied to, hence the anger and desire to find out "the truth".
Sadly the Palace have no-one to blame for this PR disaster than themselves, even if the reason for such secrecy is valid.