Tom Robinson passed by the Ewells' place every day going to and from work. He often did chores to help Mayella out, and over time she took an interest in him.
One day Mayella made advances on Tom Robinson, which he politely refused. Her father, who was a bit drunk at the time, came home and witnessed the scene. He flew into a rage and beat up his daughter badly, leaving bruises all over her, particularly around her face and neck. He needed a logical explanation to account for this, so Tom's aggressing her was the easiest alibi.
This was "the truth" of the story (according to the author), but I doubt it would have been printed in The Maycomb Gazette!
Tom would drive past the Ewells' house while he was going to and from work. He sometimes would help mayella do her chores. One night her dad came home drunk and noticed the scene. So to make it look more agressive;the dad beat up mayella and then reported the incident to the law.