Hacking someone who didn't need to be hacked.
If her phone was hacked, and it's a massive if considering that an ex-NotW PR is saying Payne didn't even have voicemail on the phone provided by the paper until 2009, then the most likely explanation seems to be it was the rivalry between the Sun and the NotW while the latter was being edited by Andy Coulson. After all, as Brooks herself keeps reminding us, her phone was among those hacked by Mulcaire, one suspects in an attempt by the NotW to nab any exclusives which were being worked on by the Sun without Brooks letting on to her close friend Andy. Otherwise, and bearing in mind that Paul McMullan isn't the most reliable witness, it's not completely out of the question that Mulcaire simply had it in case lazy NotW hacks couldn't be bothered finding it themselves, especially seeing he was the go-between for so much else. However out of control the NotW was under Coulson, hacking someone who didn't need to be hacked just doesn't seem to fit.
P.S. If for some strange masochistic reason you want to peruse a further discussion of my intellectually lightweight thoughts on Breivik, yesterday's post was also featured over at Lib Con.