Why did Petrosian stop playing the Petroff

At the halfway mark in the 1969 World Championship between Petrosian and Spassky, the match score was level. Petrosian only needed to draw the match to retain his title, and with his super-solid Petroff it looked like he was going to wrap things up. But in game 17 and 19 Petrosian suddenly switched to Sicilian, losing calamitously. John Saunders retraces the course of the match that decided who would defend the title against Fischer three years later.