I think this has probably been rumbling away in the back ground for a while. From the statement there is obviously lots of things said the can't proven to be lies or break any law but that went to far and as the press hasn't let up at some point he had to take action.
It's a powerful statement indeed and I think it's the right move they need to take a stand against the press it's been ridiculous.
As for why now it could just be that's how long the process takes but I think they have probably done it while on a high from the tour (incase of backlash) but before William and Kate go (incase they are accused of stealing the spot light). They also had to give bea's engagement announcement time to settle I guess