In case of a down vote on something that seems reasonable

Perhaps it merely indicates that the voter doesn't agree about the reasonableness. I neither voted on nor---until now---read your comment but I do note that in general many people write things that they (evidently) consider reasonable but which I consider utter nonsense and other have vehemently disagreed with (what seem to me) to be reasonable statements by myself and others. Neither objective reasonableness (to the extent that such a thing exists) nor the belief of the author will force another to perceive it as reasonable.

and/or is non-inflammatory

Not especially inflammatory, true. I note however that you opened with a contradiction, "No.". That has a clear meaning of asserting the falsity of its parent. If the parent is (perceived to be) correct then a negation may be considered sufficient to downvote the comment with or without reading the remainder.

it'd be informative if someone left a note as to why it was being down voted.

This is a true statement. Another true statement is "Writing declarations of downvotes can be perceived as a nuisance by third parties and promote emnity or at very least dowvnoting by the downvoted author. This is a negative consequence for the downvoter, who is not obliged to abandon his or her anonymity if they don't chose to."