Maybe it's not obvious - let's do the list again with each player's career home run totals:
What surprised me is that Norihiro Komada is tied for fifth on the list despite having hit less than 200 home runs in his career. Komada's grand slams account for almost 7% of all his home runs - that number is 5% or less for everyone else. He only hit more than 20 home runs in a season twice in his 18 year career. That's just a crazy number of grand slams for that few home runs.
Looking into this a little deeper, I learned that Komada was the first player in Japan to hit a grand slam in his first at bat in NPB. His nickname was "Manrui Otoko" or "Grand Slam Man".
Here's a 1994 Kanebo card of Komada (#027). Komada left the Giants as a free agent following the 1993 season and signed with the Baystars - I assume this picture is from the press conference announcing the signing.