The origin of disk galaxies is a central problem in galaxy formation. In the prevailing paradigm, disks inherit angular momentum from their host halos. However, deep HST surveys have revealed that disk galaxies are a recent phenomenon: disks start to appear in massive galaxies only around z~2 and disk settling in lower-mass galaxies is delayed until even later. Since halo spins do not depend on mass or redshift in LCDM, these observations indicate that the standard model is at best incomplete. I will summarize recent results from the FIRE cosmological simulations which support a new paradigm for the formation of large disk galaxies. The simulations show that the virialization of the inner circumgalactic medium (CGM) coincides with abrupt changes in the properties of the central galaxy and its stellar feedback: the galaxy settles into a stable rotating disk, star formation transitions from 'bursty' to 'steady,' and galactic winds are suppressed. Disk formation also appears to be associated with accelerated feeding of nuclear black holes, which we speculate drives the onset of quenching by AGN feedback. Future wide-field observations with the Roman, Rubin, Euclid, and Webb observatories will test the predictions of this new theory with unprecedented statistical power and resolution, especially at high redshift.