Orthodoxy has a Tradition of the Dormition of Mary wherein it is belived that she didn't die but merely fell asleep and was assumed into Heaven. Catholic teaching neither affirms nor contradicts the Dormition but we have another tradition which is written in one of the apocryphal gospels ascribed to St. Thomas but written hundreds of years a after his death.
Here is the ancient document alluded to. And the very ancient and venerable and understandable tradition it is in keeping with is that The Blessed Virgin herself didn't wish to in any way not be a perfect disciple and imitator of her Son. So if he chooses to die, she would choose to die as well.
I've got the same questions as to what it exactly means for Mary to have died, but it is a mystery of the Church and one you can contemplate here and ask about in christianity.stackexchange.hvn in the life of the world to come to get a better answer.
Full of grace, preserved from original sin, Mary never experienced its consequences, and at the end of her earthly life she was taken up body and soul into heaven, where we contemplate her as Our Lady of the Angels and Queen of the Universe.
Pope John Paul II - Angelus address on Solemnity of the Assumption (August 15 1998)
In the fullness of time, at the end of her earthly life, he would call her to follow him
completely into the reality of resurrection. That is what the Assumption is all about. In Mary assumed into heavenly glory, the little one of Nazareth is now resplendent in the divine light. In her we see our eternal destiny, the destiny of the church.
Most Rev. Peter J. Elliott, KC*HSJ - Retreat Homily - 2009
In some of the writings of the Church Fathers we find Jesus himself described as coming to take his Mother at the time of her death to bring her into heavenly glory. In this way they present the death of Mary as an event of love which conducted her to her divine Son to share his immortal life. At the end of her earthly life, she must have experienced, like Paul and more strongly, the desire to be freed from her body in order to be with Christ for ever (cf. Phil 1:23).
The experience of death personally enriched the Blessed Virgin: by undergoing mankind’s common destiny, she can more effectively exercise her spiritual motherhood towards those approaching the last moment of their life.
Pope John Paul II - General Audience - June 25 1997
The last link addresses very straightforward Our Lady in connection with earthly death and probably succinctly answers the question. Catholic doctrine merely says "At the end of her earthly life" it doesn't say, "shortly after her soul left her body" or "when rigor mortis set in". We wholeheartedly believe that she suffered no decay due to the wages of sin nor would she had she remained on earth after death for 1000 years before being brought into heaven.