My brother went back after being out ~15 years. Our younger sister died suddenly about 5 years ago. I believe this was one of the key catalysts. But I think the main reason he was susceptible to being pulled back is that, in those 15 years he was out, he did not take time to replace what he had believed with a more secular, critical, and scientific view of the world. In addition, during that time, he never did any research to question the "Organization" and what he had believed.
In short, in those 15 years he had not replaced a vacuum that had remained when he left the JWs. And, when my sister passed away, he went back to the only thing he knew.