IBKR is liquidating me

I bought some bitcoin ETF (IBIT) a month ago with some amount in my account. I did not do any leverage or margin trading. Just bought few shares of IBIT with the money I have in my account. Yesterday IBKR kept sending me liquidation messages then it sold some of my IBIT shares as liquidation action. I sent to support they said my account is Reg T margin type. I am not sure what this really means . Any idea how to prevent this from happening again ? I do not want to do any margin or leverage trading. Just want to have some IBIT without worrying about any liquidation requirement.