Nah, no trigger, just happens every once in a while.
Sometimes they don´t move for ages, only to suddenly take 5 cities, and sometimes they start off besieging a city every turn without getting anywhere. The Romans are worst at this I find.in my Sweboz campaign theýve besieged Segesta 5 times and Messana 8 (!) times and still havn´t taken either of them. Stupid Romans.