It's a combination of things. More salt, for sure, but trees not near roads are turning orange and brown and dying off, as well. Mainly it's just the long, cold winter, which was colder than normal this year and with extended periods to very cold temperatures (there's a reason you don't see very many tress the farther north you go in central Canada). When the ground remains frozen for extended periods, the trees can't replenish moisture (which still happens during the winter), especially when you combine that with harsher winds, more sunlight and an overabundance of reflective snow on the ground.
Then there's the classic Winter Burn, which comes from repeated rapid temperature changes, like extremely cold overnight and then warmer but still sub-freezing temps during the day, especially bright sunlit days. You see that mostly on the south side of trees where the sun is reflected off the snow and burns of the greenery.
When trees are already weaker from the stress of excessively cold winters, Winter Burn stresses the trees even further to the point where most or all of the tree is dead or dying. Adding to that stress is the further stress of disease and insects which healthier trees can fight, but can't when they are so stressed.
These trees dying off started 2 winters ago, and the past winter didn't help much. Minnesota (and Wisconsin) need a relatively wet summer and a relatively mild winter this next year to keep it from getting even worse.
(dieing is when you use a die to cut, stamp or form something).