As reported in the Tottenham Independent:https://www.thetottenhamindependent.co.uk/news/17207901.haringey-parks-have-green-flags-suspended/
Story carries quotes from local councillor, Friends of Finsbury Park and spokesman for Keep Britain Tidy (who run the Green Flag scheme).
The Council say the loss of status is not due to the impact of hosting Wireless Festival, but is instead down to a range of maintenance and cleaning issues. Perhaps the maintenance and cleaning needs wouldn't be so acute without the footfall and associated damage caused by events such as Wireless. On the other hand, the Council would probably have less money to tackle the issues without the fees generated by those big events.