These requests typically come from a Simple DNS Plus server configured as a "super slave" server in order to automatically update its list of secondary zones.
This is a special feature of Simple DNS Plus to fully automate secondary servers.
To work correctly, this must be activated in the Options dialog / DNS / Local Zones / Zone Transfers section on both "super master" and "super slave" servers, so they "know" each other.

See also: Can I use the "Super Master/Slave" feature with other DNS servers?