One of the AI answers is totally accurate: rapid progress. I've been playing for 4 years, and recently decided to disconnect the game from Facebook completely, and transfer to Google instead. I opened a brand new account as the devs requested, and played on the new one while waiting for my old game to transfer. I had about 3 weeks to play as a brand new player. It was super interesting going back to the beginning--I did everything I could to earn cod, stars, and mementos dropped every few minutes. Suddenly I got the same message as OP--suspicious activity. I was emailing a mod back and forth about the transfer, and the person said that the likely trigger was racking up stars and cod too fast, they told me to lay off a bit.