Why? Of course to un-power all what's recyclable, and probably to somewhat defend against any attempt to enforce things like https://developers.steem.io/apidefinitions/#broadcast_ops_decline_voting_rights with its 30 day period, but of course that one is dead anyways, as it can be issued ONLY by the owner of given account (owner decides to do a self-locking). It's dead because no malicious user will ever decide to self-lock the voting rights, and I don't expect steemit or justin or exchanges to do that, especially now. Considering that the steem API probably doesn't have any "lock that bad guy" ops (that could be easily misused), then changing the power-down period now can only mean one thing: INCOMING RAPID FUND DRAIN.