Fare ghosting makes about as much sense and happens as randomly as getting ghosted by a person. E.g. it typically comes without any warning signs.
The reason it happens, though, is flight prices fluctuate constantly. When you search directly on an airline’s site, you get updated results in real time, but when flight search sites and OTAs (online travel agencies) communicate with airlines, there’s often a delay so the site might not have the most updated data.