Rebel Factions besiege settlements on both Hard and Very Hard campaign difficulty and generally look somewhat active, as with this scene from a romani campaign ( H/M ) :



I´ll admit to have given the slave faction a lot of money each turn ( but haven´t created any units for them in the area where this picture is taken from ) and suddenly there were "migrations" from Raetica and Noricum, first into Germania, then later, after having occupied the Swêboz for quite some time they turned south-east, then west again and gave me a free ( as in free of doing it myself ) cimbri/teutones invasion in 113BC.

Both with and without adding money to rebel cities I believe there is a tendency for them not to march on enemy territory unless they have more than a full stack in total army strength.