Can anyone tell my why socks are being thrown onto the pitch at Goodison?

Some fans (not sure if its Liverpool or Everton) are throwing balled-up socks onto the pitch in the FA Cup replay at Goodison Park. I even saw one hit Tim Cahill in the back going up for a challenge with Martin Skrtel. Anyone have any idea as to why?

EDIT: It's coming from the Liverpool section. Whoever picked out Tim Cahill has a pretty wicked arm, he only came into the spot after jumping over Skrtel, and the guy hit him square in the back. Any guesses as to the significance of throwing your socks? I've seen a lot of missiles tossed onto the pitch in my day, but never a sock.