According to the various stats at
http://www.bts.gov/xml/air_traffic/src/datadisp.xml, air traffic in general has been basically flat since 2002. It would seem that YVR has slightly beat the overall trend. I'm not sure that there is anything in particular that needs explaining. For example, what are you referring to in 2005-06? YVR shows an incremental growth in those years just like every other recent year.