Most of the accounts are created through Steemit, which delegates to new users, so that explains the high percentage.
And some other accounts creators also offer delegations (I know we do at Share2Steem, the accounts created by Oracle-D through steem.ninja also gets a 90-day 15SP delegation, and so on...)
As accounts created with no delegation are unusable (or almost), dApps are forced to delegate to accounts they create, or it's a user lost for sure.
But the really high percentage is explained by Steemit delegating to new users who create their accounts through steemit.com