I have found that I will get these unsolicited missions from three sources:
(1) If I have just done several missions for a minor faction no matter what my reputation is (except for hostile of course lol), I will get very low value missions offered
(2) If I am friendly with a faction, I will get low to medium value missions
(3) If I am allied with a faction, I will get medium value missions offered.
To be honest, I just delete them as soon as I get them because I am normally on a mission or doing something else, they are a pain but a harmless pain I guess. None of the missions are worth my time to deviate from what I am doing.