The TSA is explicit.
If I am TSA Pre✓® eligible, am I guaranteed expedited screening?
No, TSA uses unpredictable security measures, both seen and unseen, throughout the airport. All travelers will be screened, and no individual is guaranteed expedited screening.
Officially, the airline does not choose which passengers receive expedited screening or not; this is a function of TSA's algorithms. Part of that algorithm will randomly send Pre-check passengers through for regular screening.
The whys should be self-evident (at least, from the viewpoint of a security bureaucrat). Trusted traveler programs are not a guarantee that some traveler is not a threat, or is not carrying items that could be used to harm passengers or the aircraft, so you still need to do spot checks. You also don't want people to start thinking Pre-check is some sort of badge of trustworthiness that now exempts them from carry-on restrictions, or other rules.
The TSA does not have a strong incentive to expedite travelers. They are a government agency, and if anything, they can make the case to Congress for more funding to increase headcount more easily if long lines are in the news.