Well, the reason being is that 2000/4000 is the 13th stage, and from my guess from the structure is that its starting to get deep, and into the money. With that thought process in mind, i'd be willing to guess that this is where the tournament life part starts playing a roll. If i had Aces or kings, i'd definately isolate, just because of the fact that i don't want anyone else to steal a fat pot from me, that deep into a tournament. To me, the smooth call is just an insurance scheme to be able to fold and keep playing. But then i again, i'm probably crazy.