Still Puzzled about Nitrites

Hi. If you recall I wrote about a week ago with my koi hiding and dying.
Some of you suggested nitrite poisoning, which made sense, and when I tested
the water, the nitrites were high. I have been doing water changes, adding
salt, firing up the pump and filter, added 2 more airstones, etc. and the
water tests fine now. Unfortunately, I lost 4 of my very large, "been with
me from the start" koi. I still break down and cry a lot, especially when
I'm out working on the pond, and most people think I'm nuts, but I knew
people here could relate - they were my "babies", I raised them from only a
couple of inches long, they ate out of my hand, followed me around the pond,
etc. Of course a part of it is the guilt - I feel as if I killed them or
let them down.

But anyway, sorry for the venting, here are my real questions: why is it I
still have quite a few small fish and koi fry that survived, and only the
big ones didn't make it? I thought it would be the other way around with
nitrite poisoning. And secondly, I still need to know WHY I had a nitrite
problem this winter - it's never happened before. It would be easier to
deal with this and prevent it from ever happening again if I knew WHAT I was
dealing with. Thanks for letting me vent - nice to know there's a place
where I can cry over fish and not feel like a fool.
Sue