The relevant part of Law 12:
"Direct free kick
A direct free kick is awarded if a player commits any of the following offences against an opponent in a manner considered by the referee to be careless, reckless or using excessive force:
charges
jumps at
kicks or attempts to kick
pushes
strikes or attempts to strike (including head-butt)
tackles or challenges
trips or attempts to trip"
Note the words "careless, reckless or using excessive force". That is more than just contact. If the player says what Walker said that sounds as if he himself made the decision to fall down rather than being knocked down or even knocked off balance.