Difficult to know exactly what happened, but when I've flown to Knoxville, it's been SEA-DFW-TYS. AS doesn't serve Knoxville, so the last segment would need to be on another airline, probably AA. When redeeming for award flights on partner airlines, the lowest award level (on AA that would be MileSAAver) must be available, you can't redeem AS miles for American AAnytime awards. There may not have been any eligible award seats on the flights she wanted to take so she got as close as she could.

I think you probably mean "DFW-BNA" not DTW-BNA." I'm pretty sure she didn't fly into DFW and then leave from DTW, or was there a DFW-DTW flight in there, too?