Because it's one of the times in the year where the issues that are already there come into the surface as the pressure and tension of having to experience those issues inward come about.
The personalities and neurosis and pathologies that people get to hide while living their daily lives are exposed because it's those closest to your upbringing that helped bring these things about in the first place.
It's like a clash of all the inner children that don't get to play so at times of gathering, they're still fighting for the love and care that wasn't given properly.
So it's like little children trying to be adults yet very inmature in their pain.