As for the dramatic drop, I'd bet that starting in 1973, AL teams kept relievers in *longer* than 1 inning since their spot no longer came up in the order. Relievers regularly went several innings back then. In the mid-80's, as the overall length of relievers' stints decreased, the AL caught up to the NL in the exactly 1-inning department.

Here's a challenge for you: can you think of any stat that when plotted like this from 1954 to 2010 would produce a flat line? I.e. is there any aspect of the game that has stayed constant throughout the last 60 years? Of course, exactly flat would be impossible so what about with, say 5% variation from the average?

A quick scan of common hitting and pitching stats shows that innings/game has remained pretty much the same with an average of about 8.95 IP/G. The farthest outlier is the 9.05 average in 1957 - must've been a lot of extra inning games that year. Of course, this isn't terribly interesting or telling since there's not much you could do to the game that would result in more or less extra inning games or wins at home. I wonder if there's another simple stat that's remained constant that's more interesting.