I can't say for sure the reason, but I can say with confidence the use of Fortran never really went out of style, but many of the uses were confined to classified systems and edge type or disconnected systems. Heavy number crunching is a big deal in applications like satellite imagery processing, signals surveillance, targeting and navigation. Fortran never really stopped being king there, but a lot of the development wasn't exactly taking place on engineer's personal laptops or Internet connected systems at all, so queries for how to do stuff wasn't being captured by Tiobe metrics.

The U.S. government has been making a huge push in the past decade to downgrade as much code as possible that doesn't need to be classified and push as much development as possible into fully unclassified environments to mitigate the cost of needing a fully cleared workforce. And network connectivity is generally increasing for systems that were not networked in the past.

Trends like these could easily account for Fortran developers doing more work in the open where they can just use Google to get answers instead of checking proprietary and closed information sources.