When mobile numbers are no longer needed by their subscribers, service providers release them back into the pool of available numbers after a given period, for future customers to use. However, if the number was linked to WhatsApp or similar communication services and not de-linked before the number is returned, the next user can effectively gain access to all the previous conversations and contacts associated.
WhatsApp is aware of what they say is a rare issue and says that they monitor account inactivity, and after 45 days if the number is activated to a new device, “we take this as a sign that a number has been recycled.” A recent case happening in France was reported by The Register.