You could have had a faulty brake component... Magnet , shoe , ect.... I would assume it either happened at night when you could not see all the smoke.... Or you do not use your mirrors as you done care where you've been,,, just care about where your going!! .. Seriously , I would assume that if it's as flat as you state , there would have been smoke involved!! Either way , check that brake assembly very close... Maybe it's a good time to look at them all??? And be thankful that it did not pop!!!!!! Good Luck,, Joe