The missionary work of the majority of Protestant churches are targeted on Catholics rather than non-Christians. They justify this because they don't consider Catholics to be Christians. They are very good at going after the less educated Catholics, who really know very little about the Catholic faith. A little "real" Catholic education can go a long way in preventing such conversions.
Spiritual nourishment it is not! The so-called spiritual nourishment in most Protestant churches is little more than a quick emotional high from loud Christian rock music, sometimes with a little wild dancing, and fire and brimstone preaching that the rapture may happen any minute. In the end all the jumping, singing, and pounding preaching do little for their souls. The Holy Sacraments are true spiritual nourishment!
In my experience another reason Catholics convert to Protestantism is because protestantism is easy. You walk up to the front of the church, kneel down, accept Jesus as your personal personal savior, and you are SAAAAAAAVED. As a believer your salvation is assured. And since they are now following Faith Alone there no more need for Good Works, they just do whatever in the hell they like because they have been "Saved in the blood of Jesus!". In short, a lot of such conversions are rooted in Christian lazyness.
Ran