If you want I can explain my downvote here: The topic itself is discussed a lot on the meta of every SE site. This means you didn't really do a lot of research on the topic and while I'm not a big fan too harsh criticism (downvoting if there is one small thing that could be better) I do understand why there are people that think like that.
Also you could request an explanation for every upvote aswell... now that would be silly, wouldn't it? Downvotes are not a bad thing. Sure nobody likes to get downvoted but downvotes are a thing that keeps the SE system running.
Up and Downvotes in general are meant to show whether or not it's a "good question" or a "good answer" and "good" is really a thing of personal interpretation.
Lets bring up Minecraft as an example: How do I craft an Axe?
Now If I see a question like this I might not downvote it because I don't really think it's a bad question. It's something that a lot of new Minecraft players want to know. But I do get the people that think it's something that everyone has to know (even I do and I don't play minecraft) or that just think "We're not Google". These could be 2 reasons to downvote.
Also "mindless downvoting" (let's just call it mindless) isn't as dangerous as "mindless upvoting". There are a lot of people that just throw upvotes around as if they were Facebook likes. The problem is: If you mindlessly upvote questions, privileges are granted to users that aren't even meant to have them. If you just downvote a lot you can't really do too much damage (1 UV on an answer requires 5 DV to go back to 0 rep again). I think you don't really have to explain every downvote if you don't have to explain every upvote. Sure it's nice to have an explanation but it's not necessary.
Try to look at these "mindless downvotes" from a completely neutral perspective Look at everything that could be good or bad in the question or answer. If you find at least something that's good/bad you can be sure that it's a reason for some people to Upvote/Downvote. Also if you're getting downvoted "randomly" do the same thing and think about what could be bad in your post. Your question/answer Quality will most likely improve.