Wow, this sounds... personal.

Do you have a personal assistant or other travel arranger who might have a vendetta? A spouse, child or ex who would be familiar with your travel patterns? Most of the explanations I can come up with involve someone monitoring your email or another service like TripIt, so Occam's Razor would direct the lens toward personal acquaintances (although I'm sure you've considered this already).

If you want to take the scientific method toward it, you can experiment a bit; you could book a ticket in a refundable fare class on a relatively empty flight but not tell anyone, and see if it gets cancelled. Or you could book two similar itineraries, and make sure to tell anyone you're suspicious of about one but not the other.

If it happens again, you may also try to coordinate with the police to see if Delta can examine the call recordings and match the voice with any suspects?

Sorry this is happening to you - hope you get it figured out (and we want the full story when you do!)