Well as she has been a Labour supporter and has stood for them at elections, it's hardly a great surprise. From Wikipedia:
"Bercow... changed her political allegiance during the founding days of New Labour and campaigned actively for Tony Blair whilst campaigning for her husband (then partner) to win his seat. In 2010, she stood as a Labour candidate for the St James ward of Westminster City Council, receiving 868 votes, 1,000 fewer than her nearest successful opponent.[9] Bercow is on the approved list of candidates for members of Parliament for the Labour Party and actively campaigned for the election of Ed Balls as leader of the Labour Party. The Daily Telegraph has suggested that Bercow is politicising her husband's neutral role.[10] John Bercow has rejected this criticism stating "the obligation of impartiality does not apply to my wife who is not my chattel".[11] Bercow has also appeared on Question Time, with Andy Burnham, Francis Maude and George Galloway, where she was highly critical of the Health and Social Care Bill. Bercow has repeatedly mentioned her desire to become a Labour MP and has revealed that she would like to become the Labour candidate for the marginal Brighton Kemptown seat at the next election. [12] It was reported that Bercow was in line to stand for Holborn and St Pancras if Frank Dobson retires at the next election, and recently she has been linked to standing to become the Labour PPC for Harrow East in 2015."