Trump is on record saying something like he would have let AiG go under in 2008. Which appeals to the Republican base, yes (and probably the democrat base as well TBH), but to people who actually understand economics (like the Koch brothers) that is terrifying. AiG goes under, and then it's something like T-bills stop returning on investment, and when people can't buy US treasury bills at a stable interest rate, suddenly there is no safe haven in the stock market, and you get massive increased volatility. Possibly massive inflation, and devaluation of the US currency. (And when stocks are traded on the US dollar, that causes problems too).
Bear in mind, intelligent market analysts are predicting another stock market collapse (this time starting in Europe) and most of the banks that were "too big to fail" last time are larger now than they were back then (with something like four US banks having more leverage than the GDP of the entire world). If there is another crash, then there needs to be another bailout for the same reason there needed to be a bailout in 2008--people with safe investments like US T-bills (which a lot of retirement accounts use worldwide) are relying on at least some modicum of market stability.
Trump saying "nah, fuck it, let the big banks go bankrupt. I've gone bankrupt four times--it's fine! It's a legitimate business strategy." is...pretty terrifying to someone like the Koch brothers who have lots of money, and want that money to actually hold its value.
There's also just the factor for some right wingers I know where...they try to imagine Trump, for example, negotiating with Russia without starting war...and they can't. Now, to be fair, the persona that Trump puts on for the election, and how he behaves behind closed doors is reportedly pretty different. He's an actor, and he's good at reading an audience and figuring out how to act to appeal to them. But on the flip side, his knowledge of general governing and politics has been shockingly thin. And stuff that he's said in the primary would already affect foreign affairs--I can't imagine relationships with Mexico would be good under president Trump, even if he pulls a 180 on the building a Wall shit once he's in office.