These two sources are interesting:
http://www.jbhe.com/features/57_pellgrants.html
http://en.wikipedia.org/wiki/List_of_colleges_and_universiti...
The first source leaves little question that it's happening at top tier universities, but I'm interested in the trend and second- and third- tier schools.
edit: As another commenter points out, most schools aren't sitting on large endowments. There probably isn't a single unified cause to explain rising costs. Endowments are probably part of the story for higher-profile schools, but not so much for others.
It may be the case that some universities are pulling less from their endowments to cover certain costs, covering those costs with increased tuition, and leaving the endowment free to grow more quickly, but I'm pretty confident that universities aren't just siphoning money from tuition straight into the endowment.
I think you'd have to show that universities are spending a smaller fraction of the earnings from their endowments, rather than just that the endowments are growing.
When asking why is tuition growing, I was saying exactly what you state: that for universities that are focusing on increasing the size of their endowments, something has to cover the costs that would formerly have been funded by endowments, that "something" being tuition.
Well there's your problem, most schools don't have sizable endowments. Endowments for rich schools have performed fairly well over the last couple decades, not so much for those with smaller endowments. So if you use tuition and fee money on things that might have previously been funded through endowments while you attempt to build (or re-build), it's really the same thing.
For example, Ben Hill Griffin gave money to the University of Florida [endowed school] to expand their football stadium. Down the road in Orlando, University of Central Florida used student fees to build an on-campus football stadium. It did not have one previously -- nor a football team to fill it until relatively recently (the DI team was also built largely based on student fees).
All that construction requires fundraising and together generate lots of administrative positions, and that's where rising fees and tuition go.