I think it's more likely that they had to agree on doing this in order to have the exchanges move along with powering up all of their user's STEEM.

They knew that as soon as the SP on the exchanges was used to give back voting rights to the Steemit Inc accounts, those in turn can implement the new fork and they wouldn't need to use the tokens on those exchanges for voting anymore.

So the power down is likely added to allow the exchanges to power down quickly again once everything is "back in order", so that their users won't be too mad.