The Last Jedi spoilers
Ben (voice over during flashback): “He had sensed my power, as he senses yours. And he feared it.”
Oh my god, I just realized - Ben is afraid that Luke is going to kill Rey.
Rey is on the island with Luke, and Ben thinks Luke wanted to murder him in his sleep as a boy. Ben is trying to warn Rey to get off the island before Luke kills her. Ben was trying to protect her; he was concerned she was going to get murdered in her sleep.
And think about it - this hand-touching scene ends with Luke bursting violently into Rey’s hut in the middle of the night, and exploding the entire building. And that’s the moment the force bond vision cuts off for Ben, with that image of a furious Luke in Rey’s hut.
Ben probably thought Luke had come to kill her. I can just picture him shaking as the feeling of her hand fades.