This could be nothing but an artifact (e.g. of the way the data were collected), but if you look at Fig 1 from this post, there’s a clear break in the serials expenses (EXPSER) curve that’s not evident in any of the others. Here’s the same plot reworked to emphasize what I’m talking about:

If you squint just right you can imagine a similar but much weaker effect, beginning a year or two later, in the total expenditures (TOTEXP) curve; and the salaries (TOTSAL) curve seems to start a similar upward trend at about the same time but then levels off after 1991 or so. I wouldn’t put any weight on either of those observations though — I’d never have noticed either if I hadn’t been comparing carefully with the EXPSER curve.
I’ve added linear regression lines for the 1976-1986 and 1987-2003 sections of the EXPSER data, just to emphasize the change in rate of increase. For those of you who will twitch until they know, just ‘cos, the regression coefficients of the two lines are 0.99 and 0.98 respectively. If you extrapolate from just the 76-86 section, TOTEXP exceeds the forecast for EXPSER after about 2000.
I have no idea if this means anything, but it is tempting to speculate. For instance: when did the big mergers begin in Big Publishing, and when did the big publishing companies start the odious practice of “bundling”, that is, selling their subscriptions in packages so that libraries are forced to subscribe to journals they don’t want just to get the ones they do?

Update: it’s probably nothing; the curve simply shows an increasing rate of increase, and you can break it up into at least five reasonably convincing-looking segments with breaks at 86-87 and 94-95. It’s possible there were two “pricing events” around those times, but I think this is most likely just an illustration of what can happen when you look a little too hard for patterns in your data!