I don't think Sun is buying it, could be but, it's probably mostly the general market, indexes, random traders, upbit, etc.

Looking at Hive there's very little powerups from the outside, so it's clear that people who buy it very seldomly read more about it or decide that it's not worth the staking so they may only buy it for its liquid form or decide not to because of all that.

I guess the same with Steem, little buyers but probably less selling of it than Hive cause nothing is happening there except for stakeholders growing their stake.