Let's have fun with it and play Dictionary. 3. to shine or glow. 4. to spread outward, as the end of a trumpet. For the Iridiums we can rule out flash. Iridiums ramp up to full brightness gradually and ramp down.
Glint is in the running too.
If the satellite's line scanner will look in the direction of the sun when it's low on the horizon, then we assume sun glint is possible.