Comment by Eun he on Selena - Chapter 110

@Eun he from my understanding it's because Eizer fears that once Victor gets a hold of the secret of Serenity he will be able to destroy Serena, her empire and Eizer himself. So the divorce is meant to distract or rather drive Victor's attention away from Serena and her empire.
Also Eizer made a promise to Serena's grandmother that once things are better (serena learns to manage the empire)he will divorce her.....

@Eun he for the original plan it was because of their agreement of eizer and serena's grandmother(agreement of eizer getting to use that high serena's family's nobility power while agreeing to help make the serenity family go to higher place  to protect serena in the future since grandma knows she can't be alive for to long) but now that serena and eizer fall in love, eizer gets another reason why he must have to divorce serena to protect her and her family since even thou his now a serenity his still the son of the enemy and still part of the graysons since his not legally disowned by their father, inshort victor can still borrow or even associate his evil plans-loans to banks, using eizers name to gain connection and be a bridge to serenity family., to eizer since like I said they're siblings so to cut/halt victors plan, eizer has to divorce serena to also cut his connections to serenity's massive wealth and power living the graysons bankrupt and be buried in debt which also what eizer's "revenge" plan

sorry english is not my native language but hope that helps