For refusing to subsidize high risk corridors that would have alleviated the problem.
http://www.salon.com/2017/03/22/how-republicans-quietly-sabotaged-obamacare-long-before-trump-came-into-office_partner/
When the ACA was rolled out, telling insurance companies that they had to insure anybody who signed up, regardless of previous conditions or sickness, everybody realized that the insurance companies would probably lose money in the first decade or so, until previously-uninsured-but-sick people got into the system, got better, and things evened out.
To get the insurance companies to go along with this danger of losing money, the ACA promised to make them whole for any losses in any of the first decades years. At the end of each fiscal year, the insurance companies merely had to document their losses, and the government would reimburse them out of ACA funds provided for by the law.
The possibility of their losing money was referred to as the risk corridor, and the ACA explicitly filled those risk corridors with a guarantee of making the insurance companies, at the very least, whole.
And then something happened. As The New York Times noted on December 9, 2015, A little-noticed health care provision slipped into a giant spending law last year has tangled up the Obama administration, sent tremors through health insurance markets and rattled confidence in the durability of President Obamas signature health law.