One circumstance is when you put your sim card in a different phone. The new phone recognises that you already have a WhatsApp account, as it's tied to your pho

One circumstance is when you put your sim card in a different phone. The new phone recognises that you already have a WhatsApp account, as it's tied to your phone number, but it doesn't have your private key, so it will generate a new pair and start exchanging the public part.