It's a common verification process if
1) you are playing with larger quantity of crypto, or
2) Suspected abusing their promotions like a bonus hunter, or
3) Possible exploiting of loopholes in their games with multiple accounts
4) Huge withdrawal immediately after a big win.

When you provide your KYC, make sure you are consistent with the ID you provide. i.e. if you use ID card to do KYC in Binance, don't use a different ID like passport in  Stake.com. Proof of funds will most likely include a visible bank statement with your name on your provided ID. It usually works for me.
Do read through their terms and condition if you want to be sure this never happens again.