What would cause an account check with compliance?
Anything that a casino could regard as suspicious behavior. Although it might be worth noting that the term "suspicious behavior" is usually not defined by any online casinos for the purpose of keeping their reasons a bit "fluid". In the case for example that someone is moving money in ways that could be seen as money laundering or if someone is using a bot to try to cheat the system or the player logs on with different IP addresses everytime...
The casinos need to make sure they are not involved in any illegal things like allowing players from forbidden countries to play on their casino or having their casino used to launder money.
To bad for them if they allow that because they maybe get sued for what those frauds do using their casino. And I guess such activities has been lessen especially with those huge money since many casino are now requiring KYC which is avoided by criminal to participate on. Casino really need to take counter action with this if they are allowing this then for sure they are thinking to do exit scam to all of their gamblers and they just wait for more huge balance to pass on their casino.