What causes my secondary zone to update on your name servers?

Your secondary zone updates if any of the following conditions are met

1. UltraDNS's AXFR servers receive a notify from client's primary name server
2. A force zone transfer is conducted from the UI
3. The refresh value runs out on the zone's SOA value and the zone's SOA serial on the primary is higher than the zone's SOA serial on the secondary name server