Agrarian, that was very nice, and it simply reinforces what I had said (less elegantly) earlier. However, one might ask the same question of why would Jesus die, for He was free of any sin. The answer I got in the past on this question was simply: because He chose to.
There are of course differences, since Christ was conceived by the Holy Spirit and was both God and man. The Theotokos was not.
I'm not sure that I've noted that the Fathers speak to this in detail, since it enters the realm of theological speculation: what would have happened if the Jews and Romans of Christ's day had chosen not to kill him? -- as they certainly had the free will to choose to do or not to do. Would Christ have eventually died of old age and then been resurrected? Again, pointless theological speculation, which isn't really part of the Orthodox way of doing theology.
I don't think that it is safe to say that Christ's pre-Resurrectional body was incapable of corruption -- it obviously wasn't. He suffered thirst and hunger, he suffered pain, he bled, he sweated. None of these things are characteristics of a glorified body. In fact, if his body weren't just like ours, then he wouldn't really be truly God and truly man.
There are many, many stories in the lives of the saints where saints voluntarily die at a time that seems to be of their choosing -- in perfect concordance with the will of God. They are destined for death, are ailing, but to a certain extent are able to receive communion, say their last words, and then give up their spirit to God.
I have no problem in believing that something of this sort happened to the Theotokos -- I believe she probably did exercise some will to give up her spirit at the end of her life. But she knew the end was coming and inevitable, so this control was not absolute.
Christ, on the other hand, controls the winds and waves, and is the creator of the universe. He raises the dead, heals the sick of every infirmity, and told his disciples at the time of his Passion that he could call down legions of angels to aid him if he chose. In short, Christ could have chosen, so to speak to heal himself and continue to do so indefinitely. In that sense, Christ did voluntarily die, and the Scripture says that he gave up his spirit. The soldiers were surprised that he was already dead.
But he was born with a body that was just like ours. He also was tempted in every way that we were -- and it was real temptation -- otherwise how is he fully God and fully man? The Scriptures simply say that he was like us in all ways except that he did not sin.
The Theotokos didn't sin, and yet suffered the effects of "original sin" -- she grew old and died. I've not heard it said one way or another whether the Orthodox Church believes that Christ was born with "original sin" in that sense -- i.e. the tendency to death and corruption. This is probably because we aren't focused on original sin.