It sounds like Apple had deliberately embarked upon a legal Denial Of Service attack. Qualcom produced documents [0] showing that apple actually had a plan to weaken Qualcom over a 5 years period, a plan that included forcing them into extensive litigation on multiple fronts.

I imagine the revelation of such a document played a significant part in driving Qualcom to settle. That, and their own suit which basically said, "Qualcom's licensing, that we agreed to, isn't fair" (yes, I know that drastically over simplifies the issue, but it is the crux of a portion of the dispute)

[0] https://www.washingtonpost.com/technology/2019/04/16/apple-q...