I once learned about something similar.For what it's worth, I'll share it with no guarantee that this is the whole story.
I once toured a cave whose ceiling was covered with that kind of formation. Our guide said it was called box work.
My imperfect memory of the explanation: ages ago, fissures developed in the rock. The fissures got filled with (molten?) rock of a different type.
The river that flowed through the cave caused erosion. But the filled-in rock was the harder of the two, so it remains while the softer rock around it has eroded away.
