AFAIK, there's a pre-shared key exchanged during pairing, and you then calculate keypairs based on (key, timestamp).

Rotating your keypairs in that way prevents attackers from identifying you across time. E.G. if they link a particular keypair to you — let's say at an Airport security gate — that doesn't let them know you've entered a lawyer's office a week later, because that's done under a different, unrelated public key.