Queen Beruthiel, or what to do with "loveless" wife.

[Olmer]

In his letters Tolkien has been mentioning that any character in his writing has some story. Unfortunately, he was not too diligent on giving some information about the fabled wife of the King Tarannon.
From pieces and bits we could form some very vague picture.
Margoliante (Tamar) Beruthiel was the the nefarious, solitary, and loveless wife of the twelfth King of Gondor.She has been of Black Numenorians blood, originally, probably, from Umbar. She did not like the sea, so, did not share her husband's passion, prefering to live far inland in Osgiliath, leading a very modest life style, keeping cats for a company. Seems, her husband was not too much interested in her, they did not have any children. For Gondorians taste she was too strange, and they were making the odd tales about her and her cats. That it.
How, the question: what did she do so awful, that "her name was erased from the Book of the Kings", and she has been put on the ship without crew and sent "afloat" into the ocean?
Why it was a such unusual way to get rid of your old lady?
Any speculations?