John Burgoyne and his Party Train

How did John Burgoyne go from the commander of one of Britain's best armies to defeat? 

Sources: 

Mike Duncan's Revolutions podcast season 2 episode 8, 

Our American Revolution's page on George Sackville Germain, 

Revolutionary War.net's page on General John Burgoyne, 

Mount Vernon.org's page on Sir Guy Carleton,

 and Battlefields.org's page on the Saratoga Campaign

Also, please don't judge my British accent. Thanks!