This isn’t unprecedented in iOS-device-release-schedule history, but it is definitely an outlier. It makes sense to be a 128GB model; what I am wondering is regards to timing. Why now?
Although it has been discussed as a wanted feature, it doesn’t feel like there is pent-up demand for it. The people who would be interested in an iPad with a price around $800 are also those who are most savvy to not buy mid-cycle and wait it out for the next iteration of the tablet.
My only hypothesis, or which I have a sliver of confidence in actually happening, is that a price drop is incoming for the iPad line. I would guess that the base $499 iPad would be fitted with 32 GB of storage rather than 16 GB, with the 64 GB and new 128 GB models priced at additional $100 dollar increments.