Planted too deeply? Improper watering? Congested rootball? It could possibly be a root rot but that would indicate an area of poor drainage and one would suspect the adjacent plant to be showing similar characteristics.

Root issue for sure. Possibly voles?

You might be able to tell looking at the roots of the collapsed ones, otherwise something like water molds rotting the roots might have to be verified in a lab.

In the representative photo used the smaller size of the visibly affected plant may be taken to imply a chronic infestation, such as water molds. If both specimens are the same age.