As per Chapter 2 of the Labour Party Rules, this petition is raised to demand that Alistair Campbell is expelled from the Labour Party for voting Lib Dem in the recent European elections:

'A member of the Party who joins and/ or supports a political organisation other than an official Labour group or other unit of the Party, or supports any candidate who stands against an official Labour candidate, or publicly declares their intent to stand against a Labour candidate, shall automatically be ineligible to be or remain a Party member, subject to the provisions of Chapter 6.I.2 below of the disciplinary rules."

Evidence in the video below. 

https://www.bbc.co.uk/news/av/uk-politics-48417885/i-voted-liberal-democrat-says-alastair-campbell