In my introduction post (link above) this is explained. By upvoting posts I make curation rewards. But these are not payed in liquid steem, but in vested steem power. In order to pay them out to my delegators I have to power down. So every week I power down and in one week collect the curation rewards. Then I distribute them to my delegators proportionally to invested steem.

If I would not power down, then my own sp would rise over time and that would be not zero profits.