Playing as Pontus on VH/M and Pyrrhus has inevitably attacked Byzantion, which (according to the advisor) is under my protection and thus we are at war with Epeiros.

That's all great, and such events really add to the immersion for me. Coolio!

However, after getting banjoed by the Grey Monster I tried starting again and changing my strategy. This time, both the Macs and Epeiros attacked Byzantion and no scripted war event appeared (yes, I did activate the scripting when starting the game.) A few turns later, Pyrrhus comes back and lays siege to Byzantion again and the scripted event triggers, even though we are now allies.
The same has also occurred when playing KH and Pontus attack Sinope - one time nothing happens, another time we go to war as per the script.

Is this a scripting error or do these events only trigger after a certain year or something?