What are factors leading to rising tuition costs over time?
The Delta Cost Project, a nonprofit organization in Washington DC, issues reports each year with information about college spending. The highlights of the 2011 report, which I recommend browsing through, states that:


  For the majority of institutions, increases in tuition do not translate into increases in spending. In fact, at most public institutions, tuition increases attempt to compensate for lost revenues from state and local budget reductions, but actual tuition increases are less than half of the actual reduction in state and local appropriations.


Later in the report, other sources of tuition increases are listed, including increases in benefit spending for employees (primarily public institutions). Salary fluctuations are not significant factors; in fact, spending on salaries has gone down slightly over the past few years.

For those interested, I strongly recommend checking out the highlights listed above, as well as the full report, for more details.