Why would they do that though? Like if people start associating "support Ukraine" with "get randomly attacked" then perhaps carrying out attacks could get them to reduce their support. But if the public don't think it's related, then what is the benefit to Russia? Do the Spanish government secretly know and it's a pressure tactic on them?

They do seem to come out of the woodwork quickly. Tbf I remember even before the current war, HN had a lot of Russian users - I'm not entirely surprised they would naturally defend their country, even if they aren't oblivious to what is happening.