Have to see how he walks out of the stall tomorrow. They can drift because something is chronically or newly sore or injured, they are weak on one side (muscle-weak, one-sided, crooked), they are simply leg-weary/tired, the jock is unbalanced a bit in the stretch (they all ride with left stirrup longer) and they are moving over under the jock\'s weight, moving away from the whip, away from the crowd, trying to find the rail as they are taught to do in the am when they gallop (they gallop in the middle and drop to the rail to work, so it\'s instinct to go to the rail).
And I\'m sure there\'s more than that
Just like you and I, acute injuries are painful and swell up immediately, and other insidious things don\'t hurt until the next day.