Some have either been taught to, or have developed the habit of, sliding their hand forward on to the reverse thrust levers once the thrust levers have hit the idle stops, even if they are still in the air. I guess they feel they need to be ready for a super fast deployment of reverse thrust. I have had to tell F/O's not to do it on a number of occasions.
Those that do this & then hold pressure on the reverse thrust levers to get immediate deployment once the interlocks release (as they are supposed to), will find that they will get deployment while still in the air. I have yet to discover if they are not aware that the reversers have deployed before they were on the ground, or if they are deliberately deploying them in the air.
Your hand is supposed to remain on the thrust levers until the aircraft touches down & only then be moved to the reverse thrust levers for reverser deployment.