Actually it was young Azula who first broke the news of Azulon's dastardly orders. So far what we do know is this:
1. Azulon grew irate w/ Ozai when he suggested Azulon revoke Iroh's birth rite as Azulon's successor.
2. To punish Ozai's brash request, he was commanded to kill Zuko, so he'd know the pain of losing a firstborn son.
3. Azula overheard the order and tauntingly warned Zuko of his pending fate.
4. Ursa caught wind of what was to transpire and confronted Ozai. In a compromise to spare Zuko's life, Ursa likely volunteered to kill Azulon.
5. As for Ozai eclipsing Iroh as the successor to the throne, I imagine he forged a document shortly before Azulon's death to ensure he'd rule the Fire Nation rather than his elder brother.
If we do see Ursa later this season, I'm certain all will be revealed.