I'm far from an authority, so I could be wrong on some parts here, but from what I've read among multiple sources, my best guess is this:
Among the overall agreement between Bethesda and Interplay, three parts are relavent here:
1. There was an initial agreement where Bethesda was licensed to make 3 Fallout titles, but Interplay kept the rights to the Intellectual Property (IP).
2. There was an "update" (for lack of a better word) to the agreement that consisted of Bethesda purchasing the IP for $5.75M with a condition of the sale being that they back-license Interplay to make an MMO in the Fallout setting. (Without such a condition, the total cost of purchase would've been tens-of-millions more). There was also a clause in this agreement that kept the original agreement intact in order to cover all points not addressed in the updated agreement documents.
3. The terms of the MMO license themselves require Interplay to meet certain financial and production requirements by a cetain deadline. If they fail to meet these requirements, Interplay loses the right to use Fallout content in their MMO, and Bethesda keeps the IP free and clear.
Bethesda's argument is that Interplay didn't satisfy the requirements needed for the MMO license, and that Interplay used Fallout IP content that wasn't allowed by the MMO license agreement. Interplay claims they have evidence that shows that all requirements have been met, and they had full rights to use any of the Fallout IP content. If their evidence doesn't support that they met the requirements and content allowances, they lose the MMO license, and Bethesda keeps the IP.
Interplay is claiming that Bethesda, through various failed injunctions and restraining order attempts against their investors based on contradictory and illogical information (per the judges that denied the motions), is attempting to sabotage Interplay's efforts to meet Interplay's end of the MMO license agreement (they're acting in bad-faith). Also, considering Bethesda's actions, they're also arguing that the license agreement isn't worded specifically enough to be legally sufficient based on contract law.
Per Interplay, based on these two points, the license should be made void. Furthermore, because the MMO license is a condition of the full-scale purchase of the IP (Part 2 above), if Bethesda's actions cause the MMO license to be voided, then that puts Bethesda in breech concerning that portion of their purchase agreements as well, in which case that could also be voided.
That leaves the original agreement where Interplay owns the Fallout IP and Bethesda can make three fallout games, and Bethesda would have to pay them royalties (minus their initial $5.75M payment for the IP) for FO3, NV, and all DLC, plus a not-yet-made third Fallout game, plus possibly some form of court costs.