What causes a tree on a dwarfing rootstock to suddenly start growing into a
full size tree?

I once had a similar experience with a semi-dwarf apple. You inadvertently
allowed the ground level to rise, by mulching or some other process, or buried
the graft too deep. The full-size scion put out roots and the dwarfing stock
disappeared.
Iris,
Central NY, Zone 5a, Sunset Zone 40
"If we see light at the end of the tunnel, It's the light of the oncoming
train."
Robert Lowell (1917-1977)