Firstly, a wind played with the kings hair, and stirred his heart somewhat from the dread he felt, THEN "there was a flash, as if lightening had sprung from the City. For a searing second it stood dazzling far off in the black and white, its topmost tower like a glittering needle; and then as the darkness closed again there came rolling over the fields a great
boom ." Then suddenly, he became brave and battle hungry, one can only assume that this was sent by Gandalf, using his Elven Ring to stir the hearts of the Rohirrim. That, coupled with the wind, was what kept him from running off!
