This thread caught my eye because I'm one of her constituents. While there was a large gap between LD and Con at last two elections, I believe it was a lot closer in 2010, and since then voters have probably been "punishing" LD for being in coalition, allowing Labour to move into second place.
It's a very pro-Remain area, and so I think it is quite credible that LD could take it from the Tories - if Lab get squeezed and enough Remain Tories are convinced to switch. I can't decide whether if Allen had decided to stand this time that would have helped LD get votes or not - on the one hand, I have a bit of resentment that she helped the Tories get into power in the past, on the other hand, I am impressed that she has stuck her neck out and taken a big risk with her political career.
Despite not being a Tory supporter, I have always seen her as sincere in her beliefs and trying to do the best for the country. I think she was a bit naive about the "nasty" side of the Tories and their attraction to Brexit. It is clear that she has suffered some terrible, unacceptable abuse (there was a court case, mentioned by a PP). So, I find her two reasons for leaving disappointing but fairly credible.