As some of you who've read my interpretations of prophecies before know, I hew to the idea that "dragons" in prophecy are not literal dragons, but refer to Targaryens.
We've seen visions unfold this way a couple of times in the Dunk and Egg stories:
1. In "The Hedge Knight," Daeron Targaryen (Egg's brother) has a vision of a large dead red dragon falling on top of Dunk, but he survives. The "red dragon" ends up being Baelor Breakspear, who took a blow to the head in a Trial of Seven.
2. In "The Mystery Knight," a fiddler (Daemon II Blackfyre) tells Dunk that he's seen a dragon hatching out of an egg at Whitewalls. This turns out to be Egg/Aegon V, who comes through a Blackfyre Rebellion with new fierceness and maturity.
In both of these cases, even though the dragons in the visions were actual dragons, when the prophecies unfold, the "dragons" are Targaryens.
Flash forward to Aegon V's reign. Jenny of Oldstones brings the Ghost of High Heart to court. We know that she was the one who told Aegon V that the Prince That Was Promised would come from Aerys and Rhaella's line. Not only does this set up the ASOIAF narrative, it sets a precedent for Aegon holding the woman's visions in high regard and listening to her.
We know that Aegon, Dunk, Duncan the Small and (I presume?) Jenny died in a fire at Summerhall and that the tragedy more than likely had something to do with hatching a dragon's egg. We're told that it was Aegon's decision to let his sons marry for love that culminated in the tragedy (I believe it's referred to as sorcery, fire and treason in varying combinations). I think this is a nod to Duncan the Small (the Prince of Dragonflies) falling in love with Jenny of Oldstones, which caused him to give up his crown for her (why would he have to? question for another time) and inadvertently led to the Ghost of High Heart's presence at court and her visions being taken seriously/literally. When Arya encounters her, the Ghost tells her that she's tasted grief at Summerhall, suggesting that at the very least, she was involved, and might even harbor some culpability.
So what happened? (Edited for clarity because apparently it's not that obvious that this is a theory) I hypothesize that The Ghost of High Heart told Aegon V that she saw a dragon hatching out of Summerhall, maybe even "out of a fire" at Summerhall. Like the earlier examples of dragons in prophecy, this vision would have shown its seer a literal dragon, but not actually referred to one. Having already listened to the Ghost's advice about Rhaella and Aerys' marriage, Aegon again took her at her word — her literal word — and attempted to hatch an actual dragon at Summerhall. It obviously failed, the keep burned to a ruin and a lot of people died.
That same day, Rhaegar was born to Rhaella and had a deep-seated melancholic connection to Summerhall for the rest of his life. Rhaegar was the dragon that hatched out of Summerhall.
Other than being a theory as to how and why the Summerhall tragedy occurred, what does this matter? I think it's threefold: It would be yet another instance of "dragons" in prophecy referring to a Targaryen and not a literal dragon (which I believe has implications for AA/PTWP), it broadly shows the danger of taking any prophecy at literal value, and it further illustrates that when it comes to literal dragons, the Targaryens are unstable and prone to nuttiness, even a saner one like Aegon.
I'm pretty sure that the Dunk and Egg stories will end with the Summerhall tragedy, so this should be either confirmed or debunked in good time. I think it's interesting that, within these novellas, we've already had multiple instances where "dragon" equals "Targaryen." Those instances could be a setup for the Summerhall narrative in this particular set of novellas, as well as a clue as to how to interpret dragon prophecies in the main ASOIAF storyline.
