It's about a buy-sell agreement signed by the original bandmembers when Steely Dan was incorporated in the early 1970's. It says that whenever a member of the group quits or dies, Steely Dan purchases all of that member's shares in the group.
According to the lawsuit filed in court yesterday Donald Fagen had recieved a letter from Becker's estate four days after Walter Becker's death saying that the buy-sell agreement from 1972 no longer was valid and that Becker's widow wanted 50 percent ownership of Steely Dan.
UPDATE 28 NOVEMBER 2017:
Walter Becker's Estate Responds to Donald Fagen's Steely Dan Lawsuit