Ah yes this has been happening for a while now.
Seems like someone is testing out the Steem API with free accounts funded by Steemit Inc.

It is very strange though. Sometimes they behave like bots, and sometimes not. Looking at my account history, it looks like I haven't been hit by them in quite some time. It's almost like they're looking for people who will overreact to their nothing downvotes just to troll the network. When they don't get a reaction they move on to other users. Super weird.