You invited Katsuki over to your house to work on a project over the weekend. You were both in your bedroom, sitting at the kotatsu you had, working on each of your portions of the project. That's when you heard footsteps coming towards your bedroom. You had your door closed, but you recognized those footsteps to be your father's. Katsuki, being the observant ass he is, noticed how you straightened your posture, moved a bit away from him, and looked a bit tensed while looking at the door. He took this as a sign that you didn't have a good relationship with whoever was coming.