Headcanon: After the Nightmare Moon debacle, the Royal Guard formulated “rogue god disaster plans.” They currently have thirty-one different doomsday scenarios in place, one for each possible combination of one or more princesses and/or Discord going rogue.
 
When his engagement was announced, Shining Armor had his knowledge of the plans that included Cadence erased from his mind, so that she couldn’t form counterstrategies. When Shining became Prince-Consort of the Crystal Empire, the others were removed from his memory as well, so that he couldn’t use the information therein to plan a coup against Equestria.
 
Celestia and Luna suspect this is the case, but have no proof. They regretfully acknowledge the necessity. Twilight helped write the plans, but given her ties to Celestia, her memories were erased before the plans were finalized. Cadence has no idea. Discord is fully aware of the plans’ existence and details and wholeheartedly approves of the paranoia that inspired them. He has not yet decided if he’ll play along when if he decides to go back to his old “for the lulz” mindset.