The general consensus is that the liners were not pushed all the way home when the engine was built. Subsequent machining of the block deck took the liner down level with the block surface.
Overheating then allows the block to expand away from the liner and for the liner to be able to move.
When I bought my second-hand P38A, I could hear ticking from the engine when it warmed up but we didn't know the cause. This was before the slipped liner problem became known so we didn't know to look for it when we stripped it down to look for a blown head gasket (the engine would suddenly boil and spew out the coolant).
We now surmise the ticking was from the liner moving in the block.

