To illustrate why a will might be contested, let me tell you about my grandfather’s will, which was not contested – rather, it was ignored, by the common consent of all heirs.
We all lived in a two-family house – the sort of older frame structure in which two dwelling units are built side-by-side within the same structural framework. My grandfather and grandmother originally bought this back around World War I, as tenants by the entireties. Soon after my parents married, they sold half of it to them, again as tenants by the entireties. But both my grandfather and his lawyer were more than slightly senile, and simply drew a line down the middle of the house. Some years later, my father attempted to take out a mortgage on “his half” and discovered that what had been done was contrary to the city zoning laws. Back to another lawyer to rescind the illegal deed and rewrite it as my grandparents having sold an undivided half interest in the full structure to my parents, the intent being that each couple should occupy one of the two dwelling units.
So my grandfather dies, leaving my grandmother, my aunt, my father, and myself (age 10) as potential heirs. Under tenancy by the entireties, his half interest ought to pass to my grandmother, right? Well, remember his senile lawyer?
Grandpa wrote a will which assumed he’d outlive grandma (she was 3 years older than he), effectively disinherited her (because they assumed her to be already dead), left the half house (line-down-the-middle version) which he’d already illegally sold to my father to him as his share, and left his cash estate and the other half to my aunt (who had been caring for my grandparents for decades). No mention of grandma, no mention of undivided half interests, just the oddball result described.
My parents and my aunt sat down (grandma was effectively incoherent by then) and agreed they were just not going to ever mention that will as having existed, and treat his estate as intestate, with the understanding that any liquid assets left would go to Aunt June instead of being divided between Dad and her. They called me downstairs and told me what they’d decided, and why, since I was the only possible other heir. When grandma died, they worked up some sort of covenant where title to the house was distributed 50% Dad and Mom (what they’d bought back when), 25% Dad as grandma’s heir, 25% Aunt June as grandma’s other heir, Aunt June to have life tenancy of her half pf the house.
In a sense, it was the heirs conspiring to thwart a will. I prefer to look at it as bringing order out of chaos, and the results everyone, living or dead, had actually intended.