Common for AA in DFW, ORD as well. In the case of AA, it is attributed to schedule "banking", where flights arrive in concentrated waves, believed to make connections more efficient (an argument for another time and place).
AS has made a concerted effort to improve its schedule performance, boarding earlier, pushing back before scheduled departure - particularly for first AM flights - inevitable to arrive early when things go well.
I MUCH prefer to sit on the tarmac waiting for a gate than circling in the air, still common in the highly congested northeast.