There are quite a few people with a lot of SP that are constantly powering down. They aren't powering down to leave the platform though.
Since we currently get paid in half Steem and half SP they are simply liquidating some of their assets.
As far as I can tell most people with bigger accounts are attempting to maintain their current level of SP while siphoning some Steem off in other directions, possibly towards EOS or Bitcoin.
I wouldn't worry too much about it. It's part of the process.