BGP has been deployed in Internet for more than a decade. However, the events that cause BGP topological changes are not well understood. Although large traces of routing updates seen in BGP operation are collected by RIPE RIS and University of Oregon RouteViews, previous work examines this data set as individual routing updates. This paper describes methods that group routing updates into events. Since one event (a policy change or peering failure) results in many update messages, we cluster updates both temporally and topologically (based on the path vector information). We propose a new approach to analyzing the update traces, classifying the topological impact of routing events, and approximating the distance to the the Autonomous System originating the event. Our analysis provides some insight into routing behavior: First, at least 45% path changes are caused by events on transit peerings. Second, a significant number (23-37%) of path changes are transient, in that routing updates indicate temporary path changes, but they ultimately converge on a path that is identical from the previously stable path. These observations suggest that a content provider cannot guarantee end-to-end routing stability based solely on its relationship with its immediate ISP, and that better detection of transient changes may improve routing stability.