If people face demands and pressures at work that don't match their knowledge and they're unable to cope, that can aggravate an existing mental health problem or even lead to poor mental (and physical) health. However, they can also be independent of each other.
Employers are responsible for managing and preventing workplace stress by supporting staff and improving conditions. At the same time, they should take action to help individuals with pre-existing mental health issues which are unrelated to their job.