Personally I think it's one of two things, possibly both:
Firstly, she has her own centre-right bias which despite her training as a journalist and the responsibility of her role to stay impartial, inevitably rears its head occasionally. That's human nature and it is incredibly difficult to avoid this, I'd argue that it's actually impossible unless you intentionally play devils advocate occasionally. She also obviously has absolute contempt for Corbyn and his politics and that kind of strong emotion will inevitably impact on her reporting.
Secondly, as a journalist she knows the value of networking and harvesting key sources. As such she is going to value her access to Boris Johnson, the Prime Minister, very highly. She's been cultivating that trust and access for years now hedging her bets that he will eventually get the job and she was right. She's doing all she can to retain that access and sadly that has resulted in her trading inside information for favourable reporting.