Semi-autos often jam when fired from a pocket. Either the slide is prevented from moving, or the extracted case cannot clear the ejection port.
Revolvers with exposed hammers often get cloth in the hammer area when an attempt is made to fire from a pocket, preventing the gun from firing.
You can view tests of this on youtube.
You can also view people holding their slide in place with a thumb and firing the gun, to demonstrate that it doesn't take much to stop the gun from cycling.
The guy might have had a jacket or coat on as well. If he's seated with a jacket on, then that would also have helped to muffle the sound.