I still stand by that being on boost for hours everyday in a tuned car can have this affect.
Doing this everyday is a lot different than getting a manifold red hot on a dyno once or twice or on a track a few times a month.
Didn't everyone agree that heat wrapping etc will cause cracks over a long period of time because of the heat, in the heat wrapping thread. Same principal applies.
The manifolds are worn and old now, shrink and expand with every drive. It's bound to happen