How to troubleshoot a MAC flapping between switch ports. (Cisco)

With much assistance from Ron Maupin in the comments I was able to resolve this by following one of his initial suggestions; Completely disabling DTP.

The SG300 switches do not support DTP and all trunks are run with Switchport mode trunk by default. After adding the line switchport nonegotiate to the port groups from the Core down to SW1 & SW2 then the flapping stopped occurring.

I tested this by enabling DTP again (By removing the switchport nonegotiate) command, and the flapping returned.

I do not fully understand why DTP on the core switches was causing this. Perhaps DTP frames were being forwarded by the access switches rather than being dropped? If anyone can shed any light why this fix worked then that would be much appreciated.