They haven't paid Carmack 50% of his earn out.

My point above is that Zenimax will claim that due to the issues surrounding the Oculus case Carmack failed the covenants required to collect his earn out.

EDIT - See updates to my comment, this is exactly right - Zenimax has stated that Carmack broke his covenants due to the battle vs. Oculus -=>

"ZeniMax’s stated basis for its failure to comply with the Convertible Promissory Note is a series of allegations regarding claimed violations of Mr. Carmack’s Employment Agreement and ZeniMax’s alleged intellectual property rights. Those allegations were recently put to trial in ZeniMax Media Inc. v. Oculus VR, LLC , Case No. 3:14-CV-01849-K (N.D. Tex.). Mr. Carmack was a Defendant in that case, and there was extensive testimony and evidence about the alleged conduct that ZeniMax now asserts as the basis for its current refusal to comply with its contracts."

https://www.scribd.com/document/341420632/John-Carmack-vs-Ze...