Bad endings are when the story ends for reasons outside of the logical conclusions of the narrative.
Anime and other forms of TV adaptation has this all the time; ran out of book material, made shit up for an ending, it feels unsatisfying.
In the space we all write in, it would be stuff like outside factors influencing where the story ends. Endings being rushed because the author OWES YOU NOTHING AND IS CREATIVELY DONE WITH THE CONCEPT (my god if people could just be sane enough to admit stuff like that the world we would live in), amateur mistakes stemming from authors having simply not written endings, the serialization problem, lost account passwords, and good old procrastination novels.