Well, at least I now know why you down-voted :) Sometimes I ask because I feel a comment was properly written, non-offensive and with reasonable argumentation. I put time and effort into finding the right words and thought through my argumentation multiple times before writing it down. Since English is not my mother-tongue, I'd like to know if i caused a possible misunderstanding. Hopefully others share my belief that down-voting should be reserved to punish abusive behaviour.
I understand the desire for an explanation but more often than not in my experience a comment that has quickly been downvoted to in a fit of follow-the-leader will soon be voted back up into positive territory. You just have to be patient :)
I usually only down vote people who begin or include 'i know I'm going to get down voted...' in their original post. If someone edits asking for explanation I usually give them leeway... It tells me they are monitoring the conversation and willing to engage thoughtfully.