There is no obligation for people to explain downvotes as there is no obligation to explain upvotes, this is not abuse.
Downvotes without comments are not abuse, no matter how many there are, they are a critical part of quality control on Stack Exchange.
The main reason is that the downvote itself is an explanation in itself, and the explanation can be read if you hover over the downvote button, where it says:
This question does not show any research effort; it is unclear or not useful
This is an efficient warning for any future reader that there probably are big problems with the question. 20 downvotes, as per your example is a definite signal in itself that the community has found something wrong with the question. Sure, some may be 'pile on' downvotes, but the majority would be legitimate.
This has been asked in varying forms again, again and again (a simple search finds a lot of these kinds of questions - a canonical response is Encouraging people to explain downvotes) and tend to be magnets for downvotes.