Holy Theotokos died as every man because it's the effect of the orignal sin. But because She was so holy and She was Mother of God, on the third day She was resurrected and taken with body and soul to Heaven. Orthodox focus on this falling asleep (that's why we call this feast "Dormition of the Theotokos", not "Assumption"). Some Catholics (but not all becasue tehre is no official teaching about it) believe that Holy Theotokos hasn't died so She was directly taken to Heaven. That's the difference.
So, it's an ancient oral tradition, written down a few ages after it had happenned. Why not to believe to Holy Fathers? And, we have one relict of the Holy Theotokos: Her omophorion (Holy Belt) - it was left to the apostle Thomas, who was late for this event - similary as after Christ's resurrection. BTW, according to the New Style, tomorrow is the feast of the Protection of the Theotokos and it's connected also with Her omophorion.