We don't know for sure, but the following scenario probably occurred: According to Tom's testimony, after he tried to make his getaway from Mayella, who had hugged and kissed him and was demanding that she kiss him back, Bob showed up as Tom was leaving the house. Bob may have seen Mayella hugging Tom, and he may have actually seen Mayella kissing Tom. He must have realized that Tom wouldn't have been in house without her permission. In any case, Bob yelled,
"... you goddam whore, I'll kill you."
Bob must have then beaten Mayella and afterward, he decided to blame Tom for the injuries to his daughter. Bob hated Negroes, and he must have hated Tom just for being inside his house. Blaming Tom would eliminate any legal trouble Bob might encounter for the beating he gave his daughter (though he would probably never have been charged), and he would have the pleasure of seeing an innocent Negro blamed for his own brutality. Mayella, of course, went along with her father's story, since she feared him more than anything else.