Looking at those pictures, If you don't find anything else wrong, My guess is that your input shaft might have been misaligned when transmission was reinstalled, or else something wasn't torqued on properly
The rivets that hold the hub to the disc are plenty strong in shear (the rotation direction), but with a bit of misalignment, you start introducing tension loads in the rivets as the disc spins off-axis, which they aren't really designed for. The tension loads eventually caused either the rivets to stretch, or the hub around the rivets to warp.
At that point, the rivets start taking engine torque directly instead of the hub being clamped in place by friction, and the holes in the disc became elongated, making the problem worse. The more the rivets are allowed to tilt, the more load that gets applied to them.
One characteristic of ductile materials like steel is that after they start yielding (permanent stretching, not the kind of "elastic" stretch that you get in a guitar string or rubber band), they become stronger, requiring even more force to stretch them further. (This is how torque-to-yield bolts work). Eventually, the rivets get stretched to the breaking point and shear off one at a time. If that's the way it happened, it might make sense that it didn't fail immediately.
That's my theory anyway. I'm guessing pin stretch, has been a problem for ACT - if you look on the new website, all of their solid clutch discs now have 16 rivets instead of 8, and the hub size has been increased.
The good news is that if you call and complain about a hub failure, and specifically mention that your disc only had 8 rivets instead of 16 like the new ones, they might replace it for you. Just be careful to align the transmission properly when you reinstall it, and make sure that the input shaft isn't warped or something.
