The average word count going up is likely attributable at least in part to the popularity of rap music. It didn't even exist in the 60s and 70s when the word counts were at their lowest.

Also, it seems songs have caught the same disease as movies--they are getting longer. That allows more time for lyrics.

I can't argue the point on rap music's popularity, but long songs aren't anything new. American pie is very long, Led Zeppelin's Caroselambra is extremely long, and Mike Oldfield's full version of Tubular Bells is both sides of the LP - I've yet to see any pop song get that long...