What is strange about Gregor's interaction with the charwoman?

The charwoman is the only one who is not repelled by Gregor and attempts to engage him. Gregor, however, responds in panic everytime she opens the door. After a while, she begins to open the door simply to watch him react.

Now, though, the charwoman was here. This elderly widow, with a robust bone structure that made her able to withstand the hardest of things in her long life, wasn't really repelled by Gregor. Just by chance one day, rather than any real curiosity, she opened the door to Gregor's room and found herself face to face with him. He was taken totally by surprise, no-one was chasing him but he began to rush to and fro while she just stood there in amazement with her hands crossed in front of her. From then on she never failed to open the door slightly every evening and morning and look briefly in on him. At first she would call to him as she did so with words that she probably considered friendly, such as "come on then, you old dung-beetle!", or "look at the old dung-beetle there!" Gregor never responded to being spoken to in that way, but just remained where he was without moving as if the door had never even been opened. If only they had told this charwoman to clean up his room every day instead of letting her disturb him for no reason whenever she felt like it! One day, early in the morning while a heavy rain struck the windowpanes, perhaps indicating that spring was coming, she began to speak to him in that way once again. Gregor was so resentful of it that he started to move toward her, he was slow and infirm, but it was like a kind of attack. Instead of being afraid, the charwoman just lifted up one of the chairs from near the door and stood there with her mouth open, clearly intending not to close her mouth until the chair in her hand had been slammed down into Gregor's back.