The only explanation is that they do auto upvotes. Steemit is designed in such a way that people can still curate (upvotes) without necessarily logging in. This may change in the next hard fork, but for now investors can enjoy and make some idle money.
I think it's either manual curation or smart steemit voting selling,I said manual before someone may be out of ideas for posting but can decide to be upvoting in order to be getting little curation rewards
Definitely via autovote. That's the unique thing about STEEMIT blockchain. Even a dead person's account can continue voting and earning forever.
Some inactive people keep the autovote for little reward as they are either too busy to write or no longer interested.
It's because they sell their vote. They probably use Minnowbooster or smartmarket.
They are most likely voting bots for the main purpose of community curation, they might be sold votes using vote selling system. But the most likely answer to this question is curating accounts basically for support and voting value adders to the platform
vote delegation or curation reward harvesting.