How did Yumi's father know Sachiko's father?


In the 4th OVA episode in Season 3, Yumi's father asks her what he should wear
to the Sports Carnival at Lillian. Yumi's mother then comments that he has a
sort of rivalry with Sachiko's father. When Yumi tells her father that
Sachiko's parents probably won't be coming because of a wedding ceremony they
are attending, he seems disappointed.

How did Yumi's father know Sachiko's father?

