The reason is that David Cameron's Conservative Party Conference speech was so well received, especially the part about cutting taxes that the Labour Party had a familiar panic. The BBC so often fulfils the role of the Labour Party's propaganda arm that it upped the anti-Conservative coverage. The BBC seem obsessed by how the Conservatives will fund the tax cut, I don't remember the BBC similarly asking the Labour Party how they would fund their reckless rise in public spending 1999-2007.
With news that the latest YouGov Poll shows the Conservative party with a small (1 point) lead in the main question for the first time since 21012, you can expect the BBC's coverage to become more virulently anti-Conservative.
As an aside I thought that I'd see how long it took the BBC to update their Poll Tracker with this latest Poll. It looks as though it might be quite some time as the last poll that that page includes is the Populus poll of 6 May 2014. The BBC are almost five months behind on updating that useful tool.
To be fair the BBC are developing a new Poll Tracker, well they say they are but that project seems to have stalled and reports data no later than 2013.
So there we have it, a BBC virulently biased against the Conservative Party and so incompetent that it can't update a simple database. Remind me why we all have to fund this biased and obscene waste of money?