This retreat is necessary to prevent the Black bishop from coming to g4 and trapping the White queen's rook.
14... O-O 15. h3
Further preventing any encroachment by the Black pieces on the king-side, preparing for g2-g4 and a king-side attack.
15... Bc8d7 16. Kc1b1
A precautionary move which removes the White king from the same diagonal as the White queen and from the half-open c-file. White is planning to advance king-side pawns to assault the Black king and must be wary of a possible ... Bg5 at some point down the road, skewering the White queen and king. Also, the Black side of a Sicilian usually tries to get counterplay on the queen-side, for example by doubling rooks on the half-open c-file.
This is tempting fate.
On the one hand, this releases the tension in the center, but on the other hand it forces the Black bishop back to an unfavorable diagonal. Black would be poorly advised to trade on b3, as this would leave him with a bad bishop, trapped behind his own immobile pawn center. Furthermore, this pawn advance furthers White's plan to attack on the king-side.
17... Be6d7 18. g4
A king-side attack is now well and truly underway. Black must be wary of sacs which suddenly prize open the king position.
Probably played to prevent the advance h3-h4, after which g4-g5 is a steamroller bearing down on the Black king. It has a significant disadvantage however in that it loses control over d5.