I don't think a specific reason is necessary for someone to start gambling again. Often, just a momentary urge or seeing someone else playing and having fun can spark the desire to try their luck again. It doesn't have to be anything grand: in some cases, simply watching someone else gamble is enough to rekindle that desire.
A very good point you just made, not just seeing someone else can fuel the urge but getting the news of a big win by someone can also attract someone who has stopped gambling to try again with the shape of being lucky too just like the news he just heard, quitting gambling is not yet easy to avoid going back again the person should avoid getting closer to gamblers nor listening to their success stories.