Slam is a move introduced in Tony Hawk's Pro Skater 5 by Robomodo. The move allows to instantly drop onto the rail, which is similiar to Acid Drop. If there is no rail below the skater, he will nail the ground, finishing the combo.
Since the slam move was assigned to grind button by default, it breaks the old grind control flow, where players were used to spam grind button to make sure the combo keeps going. Same tactics from now on would initiate slam too far from rail, ending the combo. Hence the multiple complaints. Gamesradar wrote about it: "There's also the new slam move, a downward plunge which feels unnecessary at best and will often ruin your combos at worst".
To adress the issue, developers announced that they're changing the slam move bind. From now on, the slam will be performed by pressing Grind button twice. However, this conflicts with Underground games setup, where double-tap grind was assigned to extension tricks.