It is important to read and understand the terms and conditions of a casino in order to avoid unwanted problems. Below I quote some points in the terms and conditions of this casino regarding Anti Fraud - maybe it can be used as a reference why an account is checked to the point of being asked for additional verification. So every customer needs to understand it properly.
4. Anti Fraud
4.1 BetBolt maintains a strict anti-fraud policy, including but not limited to:
Colluding with other players;
Engaging in fraudulent activities against other online casinos or payment providers;
Abusing promotions or engaging in other forms of cheating;
4.2 In rare cases, and only if we suspect multiple accounting or money laundering (e.g. accounts using the same IP address), we may request additional verification from the player.
4.3 BetBolt has a strict abuse policy against any player who is determined to be abusing our platform (casino or sportsbook), and BetBolt has the right to take further action and escalate the situation if a player is found to be abusing.
More things that I skipped from your quote but generally if some accounts are from a same IP address, it is very earliest red flag but later the casino actually observes more activities like betting style such as whether those accounts have same betting style, same favorite sports, tournaments and so forth. If they have all those dots connected enough, they have enough evidence to terminate those accounts and ask for more information from those users.
Mostly casinos are not wrong with their accusations against users as I read many invalid accusations from users but at the end, with many proofs, they can not denie that they broke ToS of a casino.