The assasination missions can only happen when you have an assassin close to any non-Roman family member. So that is why you get them when you have those nasty assassins sitting around.

I checked it by getting three missions in a row killing Macedonian family members in Cyrene (they had been exiled), after the third I moved my assassin far away in a fleet. No more missions like that. THen I moved him back and then I got another mission. I got a little suspicious and I kept this up (killing, moving away, get another mission, moving back, getting assassination mission, repeat) for a while until my assassin got killed.