One of the plans-to increase the medical school quota across the nation by 66 percent (2,000 more medical students a year) immediately particularly drove the young physicians into hopelessness.
From the first paragraph of the article you posted.
its doctors are among the best-paid in the world, with the average salary for a specialist at a hospital commanding $200,000 a year. Critics of the strike say doctors oppose more competition.
From this Time article.
Doctors wrote these articles to try to justify the strikes beyond the real reason.
What triggered the strikes were the opening of more positions in healthcare thus possibly effecting doctors’ bottom lines.
Edit: I’m also having trouble finding any sources that say “like 90%” of residents quit.