Hypothesis: Clowns, posing as Chinese, hack into SM-6 (Sea Dragon) launch and targeting system (development/test version) at contractor's location, linked in real-time to test missile aboard a sub preparing to head for the test range (vs. hacking directly into sub's systems, but also possible). For development/test, the cyber safeguards are less than what a fully operational, deployed weapon would have. What would normally be a routine system status ping/response is altered to transfer target data and launch timing/activation, in order to threaten/kill a certain 747 at a certain time (POTUS/AF-1 on a covert westward route or Shinzo Abe's JSDF 747).
https://en.wikipedia.org/wiki/RIM-174_Standard_ERAM
https://www.popularmechanics.com/military/weapons/a21248858/navy-sea-dragon-chinese-hackers/