(1) Most middle class jobs require only limited or mid-level skills, and they are often repetitive in nature. They are especially vulnerable to new technology.
(2) Massive LEGAL immigration has created a massive oversupply of low skill and mid-level skill workers. Politically, we could have stopped that decades ago. But now that 20 million of those immigrants have become Democrat voters, it's probably impossible to stop.
(3) The Internet, and specifically “price discovery” and “quality comparison.” Our recently acquired ability to instantly communicate with anyone in the world means we can also buy or sell almost anything to anyone in the world. I don't see any practical way to put that “genie” back in the bottle.
(4) Perhaps most important, there are vast differences in human beings. A significant percentage of the American working age population lack the energy, the intellect, and the personal conviction to adapt themselves to a rapidly changing and quite frightening economic reality.
Also the outsourcing of labor-intensive manufacturing to overseas outfits. There are much fewer good-paying manufacturing jobs that people can get with just a high school education, compared to the golden age after World War II when the USA had little competition. Those days are gone forever.