You may have answered your own question. When you stratify workload, there is an obstruction of the ability to realize the toll that it takes on the lower-tier employees. Literally expand and the supervising attorneys cannot even really individuate their time or energy to mitigating that burden, even if they do manage mindfulness of those junior attorneys.
Thereafter, junior attorneys often become the recipients of lots of work with functional completion issues, a lack of a way for the work to be appreciated, and there is rarely a way for doing a good job to be recognized, but frequently several ways for work to be criticized.
I think that it is primarily just burnout. Few people can be switched on and off endlessly and produce work that they care about, without feeling like their lives are being poured into a pit of only nominal reward but probable penalty.