This happens to me from time to time. Usually if I wiggle the trackball it stops but HTC's own advice when I emailed their customer service team about it (along with a couple of other small things), was to lock and unlock the phone... and I quote: "1. To prevent the screen from moving on its own when you don't use the device, try pressing the power button briefly. This locks the screen and prevents it from moving. When you press the power button again, you will see the last viewed screen without any changes."
