Footprints in the Morning Snow

The puzzle

On a snowy morning, a woman who lives alone woke up and looked out at her garden. Her house is surrounded by a high wall, and the gate is the only way in or out. On the snow, which had stopped falling during the night, a single set of footprints ran clearly from the gate to her front door. She had not set foot outside since the evening before. Seeing them, she began to shake with fear. Why?