Always difficult to prove these things unless you have insider knowledge which helped me win a claim with Lufthansa. However, serious weather conditions en-route, or at, or near, departure and arrival airports can lead to increased flow management measures (i.e. restrictions). Severe weather makes life difficult for ATC because aircraft need to deviate off their planned track to avoid the worst of the weather. For this reason it is safer to reduce the amount of aircraft occupying the airspace which is affected. Other less affected routings will have quickly been swallowed up by rerouters and reached maximum capacity too. In the end some guys find themselves way back at the end of the line and suffer the worst delays. That may well have been your flight.