Could some suffering be merely fate...
No not if by fate you mean:a predetermined course of events often held to be an irresistible power or agency.
I do not think that this is an Orthodox way of thinking. Some suffering is a result of sin, yes.
It is completely impossible to understand "why". This is the whole theme of the Book of Job. You want to know the ending of that Book? Basically God says,
GOD: "Hey, Job, were you there when I created the universe?"
GOD: "Then don't worry aboout why good people suffer, and bad people prosper."
There is certainly a use for suffering. It helps us on the road to perfection. How can one be humble, if he is not debased? Debasement involves suffering.
Ultimately, we will never know the "why".