The theory that something has happened that really shouldn't have due to invisible or improbable circumstances. The term derived it's name from the English Premier League team Blackburn Rovers winning the Premiership - the fact that that particular team won anything is nothing short of a miracle. It can also be used for example when achieving something highly unlikely in explaining any outside forces that may have helped.
"England won the World Cup? Blackburn Theory, that can't be right."