Game Of Thrones: 13 Crazy Fan Theories That Could Actually Be True

Robert's Rebellion came about when Rhaegar Targaryen 'kidnapped' Lyanna Stark, who was betrothed to Robert at the time. This led to Brandon Stark marching on King's Landing and demanding to face Rhaegar, and in turn Brandon and Rickard Stark both being killed by the Mad King. Thus set in motion the Rebellion, led by Robert Baratheon and Ned Stark.

However, there may have been plans to overthrow Aerys even before Lyanna was stolen away. Lady Dustin makes a reference to Rickard Stark's "southron ambitions", which is where the theory gets its name.

A big part of the reasoning behind it is that it's quite uncommon, or was before the Rebellion at least, for major houses to marry outside of their kingdom. A Stark, for example, would not ordinarily marry a Tully, but a Mormont or an Umber or such like. Hoster Tully, for example, was married to a Whent. Steffon Baratheon an Esterman. Rickard Stark married his cousin once removed, and Tywin Lannister married his first cousin.

This suddenly started to change with the characters largely involved in the Rebellion: Robert and Ned forged a friendship as wards at the Vale, and then the former was betrothed to Lyanna, while Brandon Stark was promised to Catelyn Tully.

The heads of these houses all fought together in the War of the Ninepenny Kings, so knew and trusted each other, and it seems they may have colluded to overthrow the increasingly dangerous Aerys and install Rhaegar (who was beloved by the people) on the Throne instead. Then Rhaegar went and kidnapped Lyanna, and the Rebellion as we know it broke out instead.