There is no guarantee that you will never be asked to comply KYC on an online casino even if there is no mandatory policies regarding registration, or withdrawal because every regulated casinos has their terms and rules where you can see people and residence from certain countries are not allowed to play there so if someone is winning a huge money then they have all the rights to ask about you didn't violate any of their rules to win the bet(s).
This is the truth these days. Before, we saw those casinos that were okay with no compliance prior to KYC. But today is different, everyone will be subject to it especially if you've got huge withdrawals and they'll have to ask you those certain information that they need to verify you as the genuine owner of that account and as well as them complying to their rules of operation. We'll never see today a casino that don't have the rules about questioning a user for kyc suddenly or the rights to ask the user a kyc.
But its understandable right? As you said mostly when there is sudden changes in the activity of an account will trigger the security system of a casino which may put the account on hold until the further process comes officially, yes its bit annoying but also it can save someone's money if the account accessed without authorisation from the actual user.