I wonder if the players were running in harder because the play-the-ball took place quicker and because everyone was still getting back into position defensively. If there had been a line break on the previous tackle (and there usually was), the attacking forwards were still trotting up to their own position when receiving the ball, therefore being in a position to accelerate to a faster speed in a shorter space of time. Now, not only are the defences in a line ready for the next play, so are the attackers, therefore the move starts with them standing totally still.