Something similar happened to me years back, when I was still a kid living with my parents.
One of our neighbors was looking for buried water pipes in his driveway using dowsing, in this case two L-shaped metal rods measuring approx. 10cm along the short "leg" of the L, and some 40cm along the long leg.
You hold one rod loosely in each hand, by the short leg, with the long legs extending in front of you, parallel to each other and to the ground.
You then walk slowly forwards across the area to be dowsed.
And lo and behold, when you encounter an underground water pipe, the long legs of the L swivel back towards you, almost as if something is "blocking" them from crossing the water.
After having quietly giggled a bit at our neighbor, my dad and I both gave it a try, with equally good success - and on a different piece of real estate from what our neighbor had dowsed.
My best guess is that the underground flow of water interferes with the local magnetic field, causing the metal rods to swivel.
Regards & all,
Thomas L. Nielsen