Problem engineers are not necessarily all new hires. It could also be people who were there initially but fail to transition into a role more suitable for a growing company.

A classical example, which I've seen before, is a cowboy coder who can be a useful asset in the very early stages, but who later demonstrates a total lack of teamwork ability.

Such people are even worse than bad new hires because they have a certain clout with management who is therefore more willing to turn a blind eye.