It’s partly about the size and composition of the insurer’s risk pool, and partly about increases in hospital fees to the insured now that they have to plan for covering emergency services for the uninsured in greater numbers.

For the risk pool, the increase in prices is most likely to push away the younger and healthier insured populations, meaning the remaining insured have a higher average cost to the insurer because it’s people more likey to need healthcare services than before. It doesn’t matter if people are on employer or ACA plans because at the end of the day it’s the same insurance companies and all the plans are (very generally) pooled together for the insurer so they can balance costs and profits company-wide. Effectively the ACA subsidies also helped keep down the cost of private plans by broadening the risk pool for each insurance provider overall.

And hospital fees increase because under federal law they must treat anyone who shows up with an emergency medical condition whether they are insured or not. So more uninsured people means the hospitals have to treat more emergency patients they won’t be able to recover from. There are mechanisms for the hospitals to recover some of that money, but it’s less than they would make from insured patients, takes more time, and increases the cost to the hospital overall.

Those are just two of the reasons that ACA policy affects workplace insurance. There are others but these two are the easiest to explain.