Solely from the popularity sounds unlikely, but spurred on or inspired by it doesn't sound improbable.

Seeing the way people relate to it might stir up ideas that the writer had discarded or not felt comfortable (for any reason, including brevity or ability) writing about earlier. They may not have fitted the original story, but with new ideas, might now have the strength for a full story of their own. Plus, the circumstances may have changed. Atwood wrote The Testaments as a sequel to The Handmaid's Tale and as a way of commenting on the world in the Trump age, partly spurred by the response to the TV series of the earlier book and the feelings that working on that series re-awoke in her.

Also think about how much doubt and uncertainty an author has when they're writing a book. Will it sell? Will it even be accepted by a publisher? It would be incredible if they felt enough passion to put another xx months/years of their life on the line to write a sequel at that point.