Why does my elderly mother think someone is coming in her house at night?

She might also be subject to sundowning. Sometimes people become more fearful, starting when darkness creeps in. A vulnerable person can become more vulnerable, worrying about what might happen that he/she couldn't cope with given physical limitations.

Pjkcee to help us give you a good answer we need more information. Such as does your Mother have Alzheimer's or dementia? Is she on prescription meds?  How long has this been going on? Are you at her home when this happens? If not, is she calling you at your home? How long does this last at night when your Mom thinks someone is coming in her house at night?