Aberration or trend?
Adding together all modes, the year over year increase in ridership at the Port Authority is the biggest such increase over the last decade with the exception of one period: the year over year increase between September 2007 and September 2008. My memory can't recall what might have been causing a pop in ridership back then, but maybe it was all the volunteers in town for the presidential campaigns? That is less than a guess even, just the only thing that came to mind.
Anyway, it still is a pretty negative trend in the long run, but this is what it looks like plotting out the year over year changes for every month. I didnt appreciate how well they were doing ridership-wise between 2004 and 2006, a period when Allegheny County was clearly losing population at a pretty steady clip.