I assume we're talking about the time of the Mithridates wars here.
*****GK: He's actually talking about the period posterior to 50 CE. *****
1. How did part of the Bastarnae become Romanized?
****GK: I think he means that Roman influence replaced Celtic influence at that time, affecting the Bastarnae who remained north of the Danube and in contact with the Romans.*****
2. Why would that make the other Bastarnae avoid them?
****GK: I think he's just saying that the Roman influence discernible in the material culture of the Bastarnians of the south did not reach their scattered northern remnants.*****