Business people are supposed to be unemotional, level-headed, etc. Yet they tank the markets when One of Their Own is President?

I don't think Strong Level-Headed Businessmen see Trump as one of their own.

SLHBs like conditions which are predictable and relatively calm. Under such conditions, they can use their market power to continue to build their wealth. When conditions are volatile and unpredictable, they stand a greater chance of having their fortunes wiped out. SLHBs don't like having their fortunes wiped out.

The declining price on Dow futures suggests many are trying to price their perceived downside risk from a Trump presidency into their portfolios.

It has little to do with Trump, per se. It has a LOT more to do with possible change and the unknown that accompanies that change.