My son had the same problem this past week.
My guess is simple fatigue…too much throwing and lifting.
My boys hs team just started preseason workouts, as did his travel team. So he added three additional days of training to the velocity classes and lifting he was already doing.
His pull down came in at the same exact speed as his top mound velocity. Usually there’s a 4-5mph difference. I wouldn’t worry too much about it. It should go back up with more rest and recovery.
Not sure if this is so uncommon.