Based on past observation, I think the big issue that's causing panic is/was his apparent opposition to free trade. Ever since NAFTA, both parties have nominated enthusiastic pro-free trade candidates each time - until Trump. The Democrats even trashed Nader's candidacy back in 2004 over basically the same reason. Traficant was another anti-free trade politician who was railroaded.
Over the past couple of decades, it became clear to me that any politician who was against free trade was going to be in for a rough time by the media and the other powers that be in this country. It's pretty clear to me that free trade is an extremely important issue to a lot of people, and Trump's opposition to free trade has been the primary source of panic and negative rhetoric.
From the point of view of the working classes - many people see free trade, outsourcing, and sending American jobs overseas to capitalize on cheap labor - all of that has been bad for the US economy and our diminished industrial base. This is probably why Trump's stance against free trade resonated with a lot of folks. I think that's what put him over the mark in key Rust Belt states like Ohio, Pennsylvania, and Michigan.