Maybe, it could also be something to do with the transition into normal life. This is a phenomena that can effect many sports personalities not just boxers. Some have worked their whole lives to become successful, many have no real world experience beyond the ring and when they eventually retire, this becomes apparent. Perhaps the ring is the only place they feel comfortable and the barometer by which they measure their self worth.
I agree that some are also exploited or driven back into bouts because of financial mismanagement.