How did this happen

The contract was the same for all of these years yet the benefits as % of wages has more than doubled. Continued hiring and expansion has accounted for the the increase in expense categories but that doesn't explain the benefits as % of wages.  Is this all as a result of health care costs?  The health care costs are the only variable in the equation. The next question that would follow is why has Trimet not looked for alternatives to the blue cross plan if that is the major cost factor.