I've had that a few times on Mondays following them pushing out a new software update. First time I saw that I almost had a cardiac despite not having *anything* in that account that needed margin. I called the trade desk around 0730 and they were like "oh no worries, it'll sort itself out in an hour or so" -- by the time the market opened the message was gone. Very freaky to see it, that's for sure! Even funnier was when I got that on my Roth IRA with them -- I had plenty of cash in the account, bought something, still had plenty of cash in the account, and when the order got placed I got a "possible Reg-T margin needed" alert. Again, a quick email / call put my mind at ease, but still .... not something you want to see, regardless if it's trader error or system hiccup, right?
find yourself a new broker if that is the case... I wouldnt trust that an automated alert would not liquidate positions without cause.