I can think of a few explanations:
- To really pi$$ off any JW they ever knew (the old dog-returning-to-the-vomit thing) and ensure that JWs won't bother them, ever.
- To experience the rituals involved in Catholicism, that they were never allowed to do as JWs.
- To show their support of the Catholic Church's current stance regarding pedophile priests: official apologies from the pulpit and the Vatican, acknowledgement of the pain and suffering of the victims and their families and financial restitution for the victims - something you'll NEVER see happen with the JWs.
- To go back to the family religion (if a Catholic converted to the JWs and then left).
- Because they can.
The WTS did a stand-up job of slamming the Catholic Church at every opportunity - I think a lot of times this tactic backfires and makes people more curious and willing to investigate a belief system that spawns so much hatred from "the competition".
Having said all that, it's not for me to judge other peoples' choices in belief systems - people have as much right to become Catholics after they leave the JWs as I have to be atheist.