This will happen to quite literally anyone that has to meet a contract demand. The man had to belt out 41 novels over 31 years with almost all of them paid in advance.
Having a 400,000 dollar advance over your head is a quick way to narrow vocabulary and speed things along..
The scientist mentioned that other studies have shown that vocabulary does not narrow with normal aging. Word use does not change over time.
A person's vocabulary narrowing is an early sign of dementia. It is important to recognize when this happens so that early interventions can be done.