1050+50: Big Joe button limp... common?

[Asgrow13]

So I was watching one of the 1050+50 sngs on Pokerstars and saw bigjoe2003 limp twice from the button with the blinds at 75/150. He had something like 20BBs. Is this a fairly common play for sng-ers? I never really limp from the button, but bigjoe's success is tough to argue with, so I'm curious to hear what the theory is behind this play and if its worth incorporating into my game. Thanks.