The AMOS-6 mission Falcon 9 had a RUD (Rapid Unscheduled Disassembly) event on the pad, and it is initially postulated that it was related to fueling the second stage.
We know that SpaceX revamped the GSE (Ground Support Equipment) for the Falcon 9 1.1 (since the size changed and connectors had to move) and for the Falcon 9 Full Thrust (Which requires super cooled LOX and RP1).
We know they are working to open LC-39A to this standard, and Vandenburg should be ready to go on this standard, since the first launch after the the West Coast range was reopened was scheduled for two weeks after the AMOS launch.
Additionally the McGregor, TX test facility requires similar GSE to allow for testing of the various F9 versions.
Thus SpaceX has now built the GSE for:
- McGregor, TX - Falcon 9 1.0 (Old test stand)
- McGregor, TX - Falcon 9 1.1 (New test stand)
- McGregor, TX - Falcon 9 Full Thrust (Updated to new test stand)
- LC-40, CCAFS, FL - Falcon 9 1.0
- LC-40, CCAFS, FL - Falcon 9 1.1
- LC-40, CCAFS, FL - Falcon 9 Full Thrust
- LC-39A, CCAFS, FL - Falcon 9 Full Thrust
- SLC-4W, VAFB, CA - Falcon 9 1.0
- SLC-4W, VAFB, CA - Falcon 9 1.1
- SLC-4W, VAFB, CA - Falcon 9 Full Thrust
That is a lot of iterations on GSE to build, and still have a major fault hidden within it.
Additionally, each of these systems, except maybe the SLC-4W pads, have had LOTS of vehicles run through. McGregor has tested all stages for Falcon 9 that have ever flown.
LC-40 has had all but 2 of the 29 launched stages run through it. (Two launched from VAFB). The initial Falcon 9 Full Thrust missions had a number of launch attempts where they failed to launch so they fueled, defueled, and fueled again before finally launching.
Each Falcon 9 Full Thrust also did a successful hotfire test, which required fueling and defueling.
That means the GSE has done a very large number of such cycles.
Thus it seems very interesting as what the root cause might be here, if it was GSE related.
Now it may be that we have to wait for the official SpaceX answer to this, before we can get a real answer, but I thought it would be worth phrasing the question to get it out there.