I too have flown others using my miles (both family and non family members), and I arranged the travel (booked the flights) because I was using my miles, and I was the one that knew enough about selecting flights and seats. Some of the times I was with them, other times I was not. I think it is the combination of location and class of the booked flights, the purchase of miles and no previous earned miles through flying or CC spend, and booking by a seemingly unrelated person that triggers a fraud alert. In my case, I would would have only triggered on one item, the different last names, since my sisters and nieces all have different last names.