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.
No reason. It is just what they refer to as the cues like you mentioned already. Once an addict whether to hard drugs or to gambling gets themselves close to those activities or events or people who can spark that desire back again and so on. This is the reason why it is totally okay if a gambler decides to change their circle of friends and associates if they leave gambling. What I've realized with addiction is, it needs a total abstinence for a long period of time before the brain gets itself back to its own natural control and not the influence of wanting to gamble at all cost.