Granted this may be giving too much credit to the thought that the TSA could plan everything, but I suspect there's a certain amount of attempt to load balance security lines. I'm stupidly low risk. I never get random free precheck ay my local one line podunk airport where probably 75% of the folks have it. (Apparently you can get it comped if you've got a DoD ID, and it's a military town and all) I'll get it about 50-60% of the time coming back home, typically when the departure airport is having issues moving people through lines in a timely manner- think Seattle this spring or Boston the day after the marathon.