And of course I forgot the mundane explanation, the one that I would use in the real world : it's just the too vivid imagination of an overstressed 11-yo boy on the verge of emotional breakdown after seeing the most horrible scene of his life. Nothing actually happened, but Harry's mind created the special effects that "should" come with such a tragic event as the death of Hermione.

Remember in chapter 6 : « I, Harry James Potter-Evans-Verres, do now claim this territory in the name of Science.

Lightning and thunder completely failed to flash and boom in the cloudless skies.

"What are you smiling about?" inquired Professor McGonagall, warily and wearily.

"I'm wondering if there's a spell to make lightning flash in the background whenever I make an ominous resolution," explained Harry. »

This clearly shows that part of his mind is thinking that dramatic events "should" get a dramatic special effects, and while in normal time he's perfectly aware that's not how the world works, when he's crumbling under stress, guilt and pain, he could confuse it for reality.

But while that seems a plausible hypothesis in absolute, it just doesn't feel right from a story-telling point of view.