You changed the date of a belt promotion, so it's necessary for everyone who verified you to make sure that the updated information is correct. There was a warning and explanation of this when you changed it, but you wouldn't be the first to just press "ok" without actually reading it 😄

From the FAQ:

I changed something in my promotion history and lost all my points!
If you change your belts (stripes not included), who promoted you or the date of promotion, all your votes will be up for re-verification. This is on purpose to make sure that any changes in promotion history are being verified by those who voted for you. Don't worry, everyone will automatically receive a notification and a link to re-confirm their vote once they have a chance to look through your updated promotion history.