I think he had been ill for several weeks and his office tried to hide it, for whatever reason. This increases the R edge to two seats; as I understand it, the D governor under extraordinary circumstances can set a special election for the seat in September which would leave the Payne seat open for the next five months.
The reason was they wanted to pass the big bill last week. same reason Mike Gallagher waited—to vote on bills and screw with their replacement options.