It could be one of many reasons, late arrival of incoming a/c, tech issue with the plane, crew/pilot issue, pax needed to be offloaded, and unless a member is on the flight we will probably never know.
Could be aircraft off inbound Vs44 which arrived 2 hrs late. Notice that both vs43 & vs85 left UK an hour late on Sunday then vs44 was an extra hour at Mccarran. Are the problems in Iceland causing some rerouting?