I don't think this was a mistake purely made today, it's been building up because there's regularly confusion between Hart and the central defenders. Countless times the defenders are obviously wanting Hart to come and just pick it up, but he stays on his line and they end up clearing it. This time Hart has decided for some reason to come charging out of his box, with Nastasic fully in control of the situation. Nastasic hasn't looked because this is totally out of Hart's normal behaviour, he doesn't come for much easier balls, why should he expect it this time?