Osborne made a similar announcement earlier this month, which was interpreted as a plot...
// Some supporters of the mayor claimed the chancellor's camp had embarked on a deliberate attempt to thwart any future bid by Johnson to become Tory leader, according to the Mail on Sunday. It quotes an ally accusing Osborne of trying to "destabilise Boris".
"This is nothing to do with party loyalty and everything to do with Osborne and Cameron trying to destabilise Boris. He saw it coming a mile off. They want to make sure that if the Tories lose, Boris gets as much flak as they do and can't stand as leader as a 'break with the failed Cameron-Osborne regime'," the source was reported as saying. //
http://www.theguardian.com/politics/2014/mar/02/boris-johnson-george-osborne-rift-claims-grant-shapps
It is clear they want Johnson back in Westminster, probably so they can control him. He is far too popular for Dave and Gideon's liking. They are anticipating losing the next election so they want Boris to be part of that losing team. As it is, Boris can watch them fail and afterwards say "it had nothing to do with me, we lost the election because of Cameron and Osborne's policies.". By trying to get Boris fighting the next election on that manifesto, it will be harder for him to trash it when he vies for Party leader.
I posted a question three weeks ago on Osborne's call to Boris to stand at the 2015 election. That thread is here...
http://www.theanswerbank.co.uk/News/Question1318988.html