Sometimes Probate litigation is between a new spouse or girlfriend and children from a previous relationship. The adult children feel that they are more entitled than "that floozy/gigolo" to Dad or Mom's money, regardless of what Mom or Dad said in their Will. Sometimes there is no
Will, or assets were disbursed prior to death, leading to allegations of theft, conversion, and undue influence.
More often, however, the litigation is between siblings who grew up in the same family, claiming an equal or greater right to their parents' financial legacy. Although they claim it is about "what is fair", and not just the money, it isn't. Like many divorces, the fight is really about things that happened years ago, old grudges that may date back to childhood, and a refusal to accept a decision that Mom or Dad made as to who they thought should handle their estate, or get the most property.
Every Elder at some point in time has to decide which child (or other relative or friend) will handle their affairs and make decisions for them when they become disabled, medically and legally. The "unchosen ones" usually disagree with the choice. One lament I always hear from the "wronged" sibling is that the other placed Mom or Dad in a nursing home before it was really necessary. Nobody tells me they stepped forward and offered their own home. Litigation is expensive, and the settlements do eventually come down to money. But not before the litigants have re-fought old wars from their childhood, and vented about the sins of the past.