I submitted a bug to ITA because of this a while back. I ran searches for AAA-BBB and noted the days low prices were available. I then expanded my search to include not only AAA, but also nearby airports. The prices on those dates rose inexplicably.
Here's an example, using routing codes DL+
Departing only from Chicago
Departing from ORD, MDW, MKE, or RFD
No reason those $320 fares should have disappeared.