The scalability limitations of BGP have been a major concern in the networking community lately. An important issue in this respect is the rate of routing updates (churn) that BGP routers must process. This paper presents an analysis of the evolution of churn in four networks in the backbone of the Internet over the last six years, using update traces from the Routeviews project. The churn rate varies widely over time and between networks, and cannot be understood through ``black-box'' statistical analysis. Instead we take a different approach with a focus on investigating the underlying reasons for BGP churn evolution. Through our analysis we are able to identify and isolate the main reasons behind many of the anomalies in the churn time series. We find that duplicate announcements is a major churn contributor, and responsible for most large spikes in the churn time series. Other intense periods of churn are caused by misconfigurations or other special events in or close to the monitored AS, and hence limiting these is an important mean to limit churn. We then analyze the remaining ``baseline'' churn, and find that it is increasing with a rate much slower than the increase in the routing table size.