In a fast attempt to get out of the marsh, and save the owls catch, he lifts off and power down-strokes to get elevated quickly. with relatively short stocky wings this takes a lot of power and wing speed.
He was up and away in a big hurry!
Northern Hawk Owl Downstroke
Sony Alpha α9 ,Sony 100-400GM
1/3200s f/5.6 at 400.0mm iso2000