Sometimes I think our minds panic when we get close to a new level of success. Whatever is within reach with longer fasts might just be triggering a “oh, this is new! don’t know if I can handle it” sort of reaction on some (not conscious) level. If that’s it, sometimes just trying to figure out what’s behind it (what actually am I worried about?) is helpful, because usually there’s something legitimate, some self-protective mechanism.
Or - I don’t know, it could be purely physiological! Something in some of those fat cells your body burned through @ the 72 hour mark triggered different cravings, and you just need to ride it out (and enjoy your - totally keto - feasting on the other side of the fast).