Grand Slams are scored when a batter hits a home run while the bases are loaded with runners leading to a total of four runs being scored on the play. Given that grand slams can only occur with the bases loaded, and that such a hitting situation is relatively rare, player grand slams leaders owe their place at the top of the list to good fortune more than anything else.
More about Grand Slams Leaders
Hitting anything more than a single grand slam in a season is likely to put a player among the league leaders, and hitting three grand slams could be good enough to lead a league in grand slams. No player has ever hit more than six grand slams in a season (Don Mattingly and Travis Hafner share the record). Leaders in grand slams are not always likely to also be among league leaders in home runs, showing just how fluky hitting even two grand slams in a season can be.