IMHO it(the singing/chantning) has come about as a result of the ridiculous IRB ruling that forbids the opposing team from advancing on the Haka, as has happened in the past.
Why should the AB's be allowed to throw down a gauntlet and have it unchallenged? Even former AB's have said they've enjoyed performing the Haka the most when the opponents have slowly advanced on them and taken up the mantle, often ending up nose to nose.
It is a challenge and sign of intent, therefore the opposing team should be allowed to repsond in whichever manner they wish, not just stand there and observe it from a stipulated distance.
France were subsequently fined for advancing on the Haka during the last RWC Final, however that was brought about by the IRB, not at any insistence by the AB's.
My guess is that the fans at Twickenham respond in kind for their team, as they themselves are not allowed to.
http://www.allblacks.com/News/14266/irb-confirms-haka-will-not-be-challenged