They switched to lead-free solder, ergo they had to deal with the issues inherent to lead-free solder. What part of this isn't clear?
There are reports on this very forum of AMD cards being fixed by re-flowing them. A card that goes from perfect, to broken after extended use (and after lots of thermal cycling), that is then fixed by re-flow is pretty much without a doubt suffering from microfractures.
The fact that AMD delayed lead-free parts from public consumption until well after both Intel and Nvidia had already launched lead-free parts makes it pretty clear they didn't think they were ready yet.
So basically you just said, "blah blah blah, I don't understand the technical details of bumpgate so I will just make up what happened."