The British had moved up the field far quicker than they planned on, and had spread out a bit too much, and needed to pause and consider the stubborn, oddly determined enemy ahead. The enemy had impressive guns.
The British also wanted to make sure the fleeing militia actually left the field and did not turn and fight. This proved to be all too true. Commodore Barney and his men were left alone to face many times their number. It must be noted, however, that the British did not even have all their 3000 men in action yet. One third of their force was not even yet on the battlefield! This was the 21st Regiment (but this regiment's light company WAS engaged), and the Royal Marines and British sailors were also still back in the town of Bladensburg.
Com. Barney fired his guns, and the British knew it was time to shut this small enemy force down. They advanced further up the hill into the American fire.