According to the Pre-Check web site:
Other Ways to Experience TSA Pre✓™
You may still be able to experience TSA Pre✓™ screening on a flight by flight basis.*
TSA Risk Assessments uses Secure Flight Data already supplied to TSA, which allows passengers to access the TSA Pre✓™ screening lane on a random basis. Check your boarding pass each time you fly for an indication of TSA Pre✓™ eligibility.
Guess I just got lucky this time.