Slitherine

I lost Atticus to Antigonids, the region had 75+ loyalty and no unrest. I became an old empire the turn before, and I am neutral towards Antigonids
This is the first time it happens to me, what are the rules regarding this ?
Thanks in advance !

These sorts of defections only occur for Alexander the Great's successor states. If one of them is old or decadent, regions can defect to neigbouring factions in better goverment states. It's mentioned in the manual under "7.7. Specific Rules for the Diadochi", but it does not go into any further detail regarding the mechanic.

Oh yes I always forget the Diadochi special rules
Thanks !