5 books I've read. 5 times I've waited and waited for GRRM to finally reveal what exactly happened all those years back during Robert's rebellion. Be it in a flashback scene or whatever.
He still hasn't revealed it. Oh he's given bits and pieces sure, I know its something to do with Ned's sister, and rhaegar, and his brother, and even his father, and the mad king, but he's never point blank said it.
I think I've pieced a lot of it together and I wouldn't ask but it's literally the driving force behind the entire series.
And quite frankly every time I imagine the idea of Robert and Ned as boys or in their prime as best friends in service to a younger jon arryn, I feel like it would make a great prequel, like the marauders in Harry Potter with young James and Sirius and lupin. Hearing about how close they were and more like brothers than even Robert's own brothers makes me interested in this idea every time.
Lyanna runs off with Rhaegar. Ned's family (and Robert) think he kidnapped her. Ned's brother and father go to King's Landing to ask Aerys to return her. He, being insane, burns them to death. This begins Robert's Rebellion.