WMS use a web connection and sometime this connection can be "cut" or lost. When a zoom or pan is done the WMS image data is read from the WMS server. When the WMS image is attached in multiple reference files then it causes a conflict because Raster Manager is trying to do multiple reads of the WMS data from the WMS server. The WMS image should only be attached in one DGN file.
http://communities.bentley.com/Products/MicroStation/MicroStation_V8i/w/MicroStation_V8i__Wiki/raster-manager-_2D00_-server-streaming-raster-visualization.aspx.