Economic growth tends to occur in a cyclical pattern.
This pattern is called the trade cycle and it can be clearly seen when looking at the behaviour of growth over the last 20 years or so. There is a graph showing this in the explanation section on economic growth. Recent examples include the failure of banks in Japan, and the collapse in SE Asia. Both of these have affected the UK a great deal. It would be cynical to suggest that this often occurs around election time, but it does.