I think it's the people that caused the trees to move Westward. For decades now I've traveled to Arizona for business. Thirty plus years ago, Phoenix has little pollen problems but now, after a lot of retirees from the East coast moved to AZ and started to plant the trees they are used to see. Phoenix has pollen problems left and right. I've seen similar issues in Colorado, New Mexico etc..
To the OP,
Unless you are growing a few species that will form root knees such as cypress, if your roots show and your trees are not leaning over, it is likely soil erosion.
No pictures, but I know what you mean as I have the same situation.
In some cases I think it is erosion (or soil compaction?) (Or even frost heaves?).
In other cases, the wind, has made the tree lean over the years and raises the roots on the prevailing wind side. (Or similarly, if one side of the tree has committed hari-kari and is now unbalanced. I have an apple tree that has done this 4-5 times. )
In other cases.....???? (The underground mole people "raisin' the roof'?)
Depending on type of tree and size, they could simply be growing bigger.
Pretty common for hardwoods like beech, oak, and maple to have the roots showing for several feet (sometimes with some buttress or flare). Basically, if you want the tree to stay healthy, leave it alone. Don't try to cover with dirt, piling mulch on them can even be worse, but mulching between them is probably OK. Yes it makes it a pain to mow.