I dont really agree that you need to explain what was wrong with the content. And brigading happens with comments too, arguably to a worse end.
Explaining every vote you make, or requiring a yes vote before you allow a no vote, also defeats the purpose of voting at all. I get it can feel bad to have a comment downvoted but… Like… Its really not a big deal. Especially in a system that has even less weight on karma. Sometimes the group didnt agree with you, and thats a normal part of community interactions. You shrug and move on.