To be fair, a big deal of those 15GW probably went down as a result of the initial outage: equipment is designed to intentionally disconnect completely when the grid is behaving badly, rather than trying to force it into submission and potentially causing serious equipment damage.

In reality this means you might lose, say, 1GW due to a transmission line failing, have a big frequency dip as a result, and then have 14GW drop offline like dominoes because they sense a grid frequency outside of safe operating parameters; disconnect as they go into safety mode; cause the frequency dip to worsen; and pull even more plants offline with them. If you're not careful, a small outage can quickly cascade into an entire grid going offline.