Sadly, seizures are fairly common in dogs period... not just particular breeds.  Most often, the cause of the seizures is never determined.  It is sad when seizurfes are frequent and meds don't control them, but this does happen.  I am sorry for your loss.
As for any connection with this animal being a white boxer, there is nothing (other than deafness and possible sunburns) that white boxers ar emore prone to than fawns or brindles.