This is what the running integral of the QBO time series looks like. I am pondering whether this has something to do with the disturbance I see in the model fit after 1980.
An integral acts as a low-pass filter and so will show the longer wavelength components of the QBO.
The last post in the other long-running ENSO revisited thread, shows how a phase reversal in the standing wave may be occurring around that point. It also looks like it occurs at a zero-crossing, which is the only possibility consider how much energy it would take to instantaneously flip the amplitude from one sign to another.
I have to find some examples of a metastable phase reversal occurring in a standing wave.
By luck, I found this paper:
Greenstadt, El W., et al. "A storm time, Pc 5 event observed in the outer magnetosphere by ISEE 1 and 2: Wave properties." Journal of Geophysical Research: Space Physics (1978–2012) 91.A12 (1986): 13398-13410.
I don't fully understand this, but they show a figure that shows a similar cumulative buildup and then a release as a phase reversal: