This happened to me once, and only on the outbound. I had precheck on my return. When I looked into it I found:
- The ticket had been purchased through DL.com using my profile
- My profile had remained unchanged through the process. KTN was always in there
- TSA can randomly take away your precheck for security purposes
The problem never came back. Maybe I shared something to facebook that pissed off the government, who knows?