The .co.za Registry, also known as ZARC (https://www.registry.net.za/) does Nameserver checking on .co.za domains when a Nameserver update is done.

During these Nameserver checks, the Registry ensures that the new Nameservers are in fact configured correctly for the domain name and are answering authoritatively for the domain name.
If any of the checks should fail, to any Nameserver specified in the Nameserver update, ZARC will decline the Nameserver update.
This gives the appearance that the Nameservers for the domain are reverting back to their previous values.

Should this issue occur, you should check if your new Nameservers have been configured correctly by contacting your provider.