In book 2 chapter 10 Why does Winston get caught now right before book 3 starts?

A large part of the plot is directed at the idea that O'Brien had been watching Winston; at least, since he first purchased his diary from Mr. Charrington's shop. Winston fascinated O'Brien. O'Brien felt Winston was his intellectual equivalent. Winston became O'Brien's project to watch and study. It is not until the end of part two that O'Brien feels he is ready to "re-educate" Winston.