Got same text last week. Flight booked directly on ba.com; Interestingly I've only held the ticket for the cancelled flight for one hour.
I was originally booked on the evening flight to LCY, waited until midnight and used Same Day Change to swap for late morning. When I woke up next day, I saw cancellation text (of late morning flight) received at 01:17. Originally booked flight operated normally, so it must've been that SDC'd flight that triggered it, even though I only rebooked on it 1 hour before cancellation.