I believe many people who return to gambling after quitting are likely due to several factors, such as being surrounded by gamblers, which can lead to them wavering in their resolve to recover from their addiction after seeing one or more of their friends win. This can clearly lead to their re-entry into gambling.
It's also possible that their finances have stabilized, and the presence of a steady income may make them more confident in gambling in pursuit of winning, which is actually not recommended. It's likely that very few people return to gambling, but with a different approach, such as returning to gambling with established limits.