If her tennis shoes hadn't gotten wet, then maybe her bag wouldn't have been ripped.
If she had listened to her gut feeling and gone back for them when she realized they were forgotten at Tooru's house, if she didn't ask him to bring them to school in the morning, if she checked the weather, if she texted Hajime instead, if she did anything…then her shoes wouldn't have been left behind in the storm the next day.
And afterwards, maybe she would have actually stayed home.
If she hadn't felt something snap deep within her, if she hadn't felt such a heaviness against her lungs, if she didn't feel so horribly alone back then, then maybe she wouldn't have been in such a rush to leave. She wouldn't have given into the impulse of buying her train tickets, nor would the awkward conversations with her parents have happened later on. She wouldn't have packed as hastily as she did, and would notice the way the strap of her backpack was still caught on the foot of her bed. If she did, then she wouldn't have pulled as desperately on her bag, trying to get away.
So in the end, if her shoes hadn't gotten wet, then none of this would have happened.
But then, this would be a completely different story.