Health insurance costs went up because they have to cover more. They MUST cover birth control even if you don't need it or want it. They MUST cover routine gyn exams and mammograms, whether or not the person being covered is male or female. They MUST cover childhood immunizations whether or not you have a child on the plan. Now, my premiums are going up in 2014 even though the above mandates were already met by the plan in 2013.
Another reason could be that the insurance rate went up but the employer is not absorbing the rate hike at all or at least not proportionate to how they covered their employee in the past.
The hospital that you work for may be "self-insuring", providing healthcare to their employees for a low-cost premium as long as you stay in-network (meaning use hospital doctors, the hospital lab, the hospital pharmacy, hospital diagnostic and treatment centers, etc.). They can keep their administrative costs low by acting as their own mini-HMO. Think of it like the old "professional courtesy" that healthcare professionals often extend to one another.
Any of those reasons could factor into why premium costs have gone up. Some are due to Obamacare mandates and others are simply due to the increased cost of delivering healthcare...period. Under Obamacare, the insurance companies must send rebates to customers (either the employer or the individual who purchases their own insurance) if they fail to spend at least 80% of premiums on health care. I got a check for $1.89 this past summer

and yet my premiums will still go up again next year!