The dam supporters' accident was the second instance of Oyashiro-sama's curse. The leader of the pro-dam group, Mr. Houjou, took his family on a trip to a national park. While on an observatory, he and his wife fell from the observatory deck. Mr. Houjou was found dead at the scene but his wife's body was never found. She was pronounced dead a year later due to low expectancy of survival.
The Houjou family was very poor, so the money received for supporting the dam project would've been a great opportunity for the family. It is rumored that Mr. Houjou receieved bribe money from the Ministry of Construction while he led the pro-dam group. The Sonozaki family consolidated the anti-dam group and ended up converting most of the pro-dam people to anti-dam. This led to the entirety of Hinamizawa demonizing the Houjou family for supporting the dam.