The homeowner backfilled the wall with soil, skipping the only mildly laborious step of backfilling the wall with an eight-inch-wide trough of 3/4 inch clear (meaning washed) gravel.
This is necessary not only here in true gardening country, Zone 4, where winter frost in soil causes stone to move, but should be a part of stone wall construction in warmer climes. Even in Zone 8, if you wish to construct a stone retaining wall taller than 24 inches, you really should install drainage gravel behind it. A heavy rain will turn soil to mud, and once soil turns liquid, its lateral push is remarkable. In Texas, for a wall this size, it might not wash out all at once, but it will slowly move, gaps will form, and in a decade or so it will resemble the wall pictured. Youll enjoy hammering the stones back into place with a dead blow stone hammer for only so long. And eventually, they dont go back in all the way.
For large boulders, a gravel drainage trough behind the wall is not necessary only landscape fabric, to keep the backfill soil from flowing out the face of the wall. Boulders weighing 150 pounds or more are heavy enough to stay in place without drainage gravel, even in northern zones. Plus they usually involve a bit of a setback. But for boulder walls using small boulders, 12 inches or less, setback or no, if you get up around 24 inches high, you need to create a gravel drainage backing, because the small boulders dont weigh enough to stay in place. Theyll move and collapse after repeated exposure to rain or irrigation, anywhere in the country.