The boom is consistent with enrollment growth. So, how much of the hiring is new faculty? You could ask our clueless provost Jim Bean, but the best data I've seen is from the AAUP's Howard Bunsis and his February report to the UO faculty union organizers. Bean and Berdahl's failure to give a credible response - or any response - to this report is a big part of the reason we are now unionized. (Whoops - as a comment notes, Bean was actually on "sabbatical" when the vote occurred. Lorraine Davis was acting provost.)
The increase in FT Faculty is largely adjuncts. There's been very little increase in tenure track numbers at UO:
From 1992 to 2012 the number of students has increased by more than 50% (data here) and the number of tenure track faculty by less than 20%.
Since 2008 the number of students has increased about 20%, the number of tenure track faculty about 10%.