Late presentation of ruptured type A aortic dissection preceded by several weeks of symptoms. 

Why did the false lumen rupture, and not the true lumen?

Dissection occurs when blood enters and splits the media through an intimal tear, separating the intima and adventitia. The false lumen has a pressure equal to or greater than the true lumen, reduced elastic recoil compared to baseline, and dilates as a result. This collapses the true lumen, as the dissection flap does not respond to absolute aortic pressures. The wall tension due to aortic pressures is instead completely transferred to the weakened outer wall of the false lumen, which can become so great that it ruptures, as in this case.