I can't speak for others, so what I'll say here is how I see my commentless downvotes. The downvote tooltip says "This question does not show any research effort; it is unclear or not useful".
Most often when I downvote without commenting, it's just that. Completely unclear or not useful at all. And so much so that anyone should really be able to see that from looking at the question. If the problems are more subtle I generally do leave a comment, if a similar comment hasn't already been made. If it has been, I'll upvote that comment.
This assumes a couple of things though: I assume that you as the user have familiarized yourself with the site before asking your question. You have done the minimal you should do and have read what the site is about, read about what is on-topic and off-topic, and you've researched what you're asking and demonstrate that you have.
If your question subsequently is off-topic, unresearched, or one of countless duplicates, you might not get my comment. I could downvote you and will vote to close if necessary.
In general I love helping you (the user) out, and I'll do my best to make your stay as pleasant as can be, but I don't usually feel like explaining those issues again that I feel you should have at a minimum familiarized yourself with. So you might end up with a commentless downvote from me for those reasons. (Though in practice I often still will explain or ask for a thing or two before doing so)