To submarine someone is to suddenly and unexpectedly end a romantic or professional relationship without explanation or communication.
Submarining can make people feel confused, hurt, or frustrated.
Examples:
She submarined him without saying why.
He got submarined by his boss and doesn't know why.
They submarined her and she felt very sad.
He was submarined by his friend and now he's all alone.
The team submarined their coach and now they don't know what to do.
She submarined him and he was very surprised.