I think it's the electoral cycle. Any party that wins 3 terms tends to suffer a severe backlash, not just from the electorate, but internally, that is, they become exhausted and jaded. Thus, after 1997, the Tories had a sort of nervous breakdown, and didn't win again for 18 years. Labour are going through the same thing, and I don't think it would matter whether left or right were in charge. It's partly remedial, since each party is forced to examine itself or fall apart, or both, as it has its breakdown, and in the end, this rejuvenates.