[Given Homura is what she is, she'll probably be able to sense Sakura approaching, with her immense power and generally positive influence on the place. Of course she would go seek out the place where all this dark energy is gathering, she wants to do something to change it...
And sure enough, by the time she's close to the shrine, her own jar starts glowing, releasing the positive energy in it towards it. Sakura has plenty of it added up, but it won't be enough to counter everything by herself. Still, she allows it to happen, as it's easy to see the power transfer is immediately making the darkness back away, if only a little.]