Basically British aggression. From 1812 thru 1814 the British raided US shipping all up and down the coast. There were even British bases on some remote points of land and islands. Tangier Island in the lower Chesapeake Bay was one of the British land bases. From there, the pretty much controlled everything in and out of Baltimore, Annapolis and Newport News. They not only picked over cargo, they also impressed any sailors they claimed to have originally been British citizens.
I think the reality was they misjudged American grumbling about government; mistaking internal political discord/discourse for willingness to return to the empire. I think they fully expected the locals to welcome these landing parties giving them food, shelter, horses and other aid.
Historical note about the Ft McHenry siege - first recorded use of a frigate converted to shoot rockets at a shore target. Shooting flaming rockets from a wooden ship must have taken balls.
