Also, it comes almost immediately after the government became aware of what was leaked (thanks to David Miranda). Not knowing this put them at a severe disadvantage before, as they were constantly a step behind and being continually caught lying was ruining their credibility. As others have noted, "was contained in the leaked documents obtained from the NSA by Edward Snowden" are weasel words of the highest order, and the linked article by the Guardian is clearly calling this out.

It's easy to picture a narrative where the government wishes to ham up the "national security risk" posed by these leaks, which they couldn't do effectively when they didn't know what was in them. Intercept David Miranda to find out, cherry pick some information that sounds important but non-threatening to UK nationals, and leak it to The Independent in exchange for a sympathetic story. This also explains even the nonsensical "destroyed hard drives" affair - they weren't trying to delete the data, they wanted to make a big show of "trying" to.