This topic has come up recently so I did a little digging and was able to find some interesting information on the UNT System site.
I came across this .pdf document which outlines the increase and it does specifically mention that it is for student athletics along with some other area (retention, engagement and wellness):
https://untsystem.edu/regents/agendas/13-03-26/2013-19%20Approval%20of%20Increase%20to%20UNT%20Student%20Services%20Fee.pdf
You can read the entire thing if you have a PDF reader but the key sentence is: "An increase of $1.31 cents per SCH (semester credit hour) from $13.10 to $14.41 would generate approximately $1,200,000 per year."
This is slated to begin in the Fall semester of 2013.