> I don't know the cause of this.

I think it's pretty clear when you analyze it from the perspective of Selectorate Theory (c.f. Bueno De Mesquita's Logic of Political Survival).

Basically, there's a natural tendency for political parties to bring entire classes of institutions into their patronage network, leading to extremely high polarization within given industries. The choice of which party an institution class gets aligned with may be entirely arbitrary, but you expect it to happen. It's an efficient way to pork-barrel buy votes.

E.g. the education sector is part of the D patronage network and the ag sector is part of the R patronage network. There's no inherent reason this particular selection needs to be the case, but you do expect some kind of polarization to emerge.