Do you think that this was intentional? Maybe to find reasons why they can't process your withdrawal? I think about the same too, but what about online casinos will enough credibility already? I mean very popular online casinos.
The procedure of requesting KYC after you win a bet is indeed very annoying for gamblers. But anyway casinos have their procedures for managing accounts with big wins. Of course, it does not rule out the possibility that there are people who abuse the casino to gain profit. Even if the casino has asked for your KYC at the beginning of creating an account, there is a possibility that another stage of KYC will be requested to verify your account properly.
I always prefer to use Casinos that outrightly tell their new customers at the beginning of registration, to fill their KYC forms. This is better than not saying anything by the Casino in this regard, only for a customer to win big and they include KYC requirements as part of the withdrawal process.
I have heard of someone who was complaining of such an incident that involved him bringing these documents to fill forms before his winnings would be released to him and this isn't a good practice by casinos who aim to beat the credibility test.