They are both conservatives from the perspective of classical liberalism. More specifically, they are conservatives in the sense that F.A. Hayek used the term in 1960 when he wrote the postscript to The Constitution of Liberty titled "Why I Am Not a Conservative." There he said of conservatives,

They typically lack the courage to welcome the same undesigned change from which new tools of human endeavors will emerge.... This fear of trusting uncontrolled social forces is closely related to two other characteristics of conservatism: its fondness for authority and its lack of understanding of economic forces.... The conservative does not object to coercion or arbitrary power so long as it is used for what he regards as the right purposes. He believes that if government is in the hands of decent men, it ought not to be too much restricted by rigid rules.

That description would seem to apply to both Trump and Sanders. They share a fear of uncontrolled and undesigned change, especially in the economy. This is most obvious in Trump's bluster about how America never "wins" and his desire to raise tariffs on Chinese imports and close the flow of immigrants, especially from Mexico. Economic globalization is a terrific example of uncontrolled change, and using foreign workers and producers as scapegoats for that change -- especially when those changes have largely benefited most Americans -- is a good example of this fear of the uncontrolled.

Those policies also show the much-discussed economic ignorance of Trump and his supporters, as shutting off trade and migration would impoverish the very people Trump claims to care about -- those who are, in fact, supporting him. International trade and the free migration of labor drive down costs and leave US consumers with more money in their pockets with which to buy new and different goods. They also improve living standards for our trade partners, but Trump and his followers wrongly perceive their gains as necessitating American losses.

The same concerns are echoed in Sanders's criticisms of free trade and in his claim that immigration is undermining good jobs for the native-born.