To my knowledge, operation hours of the Precheck line is solely at the discretion of the TSA. United charges the security fee on each ticket regardless of time of day (per regulations) so it's entirely up to hours. As far as when they choose to open I think it has to do with passenger flow/numbers - although I've shown up early at checkpoints before (e.g. 4:30AM) and the precheck line is not open but there are quite a number of passengers.

Last time I went to terminal A (~5AM on a monday) the Precheck line was not open, but they were essentially giving everyone the precheck treatment anyways ("don't take anything out" was the instruction). Same thing happened in ORD T1 last thursday afternoon - Precheck line was closed, but everyone was getting the "don't take anything out" treatment and went through the WTMD unless they asked for AIT or they triggered the WTMD repeatedly.