It is a mystery. That's the best explanation so far. God isn't in the habit of explaining these sorts of tragedies, and really, why should he be expected to? What was it that God told Job when Job (at the prodding of his colleagues) questioned God? He got sort of sarcastic, and turned the question around on him:
Job 38
1 Then the LORD answered Job out of the storm. He said:
2 "Who is this that darkens my counsel
with words without knowledge?
3 Brace yourself like a man;
I will question you,
and you shall answer me.
4 "Where were you when I laid the earth's foundation?
Tell me, if you understand.
5 Who marked off its dimensions? Surely you know!
Who stretched a measuring line across it?
6 On what were its footings set,
or who laid its cornerstone-
7 while the morning stars sang together
and all the angels shouted for joy?
8 "Who shut up the sea behind doors
when it burst forth from the womb,
9 when I made the clouds its garment
and wrapped it in thick darkness,
10 when I fixed limits for it
and set its doors and bars in place,
11 when I said, 'This far you may come and no farther;
here is where your proud waves halt'?....
Excellant wimpycat!
Outstanding! And pertinent!