For some reference, Big Top (in 2016 and 17) started going up around this time of year. I think Survival Games started going up mid-to-August. Mazes in permanent locations start being built earlier too.
I would imagine the reason for the comparatively early start on the FN build is down to the number of man hours available. The park will have a small Production Team, and work with UVE (presumably - they've done Trailers, SG and been involved with FN for a couple of years now), to build the mazes and zones.
A maze in a tent will need to have all the walls built and then scenic design applied once the walls are in place. They can't really do that work prior to building the maze. As such, they'd need to start early to get that all set up. If they're stretched for manhours, they'll need to start early, hence why getting Death's Doors up early likely happened.