Some automatic transmissions allow the driver to force a lower gear to engine break. It is not used as often, but can be useful in long slopes to avoid overheating the breaks and reduce wear.
I understand why: it's most efficient to drive in the highest gear even at speeds as low as 45 mph in a manual. I would imagine it's similar for automatics too.
Ford trucks with a Tow/Haul button enabled will do exactly what you describe though, especially if you tap the brake for an instant while going downhill.