Fixing a 1920 row house - hardfloor cupping/buckle/warpping
Moisture can often cause that, if a spill isn't cleaned up promptly or you mop with excessive water, for example.

Likewise, if this is over a wet basement this can happen, but I would bet the issue is from the top half of the floor (in the room).

Less likely is that there is not enough of an expansion gap between the sides of the floor and the wall, and temperature changes in the room are forcing the boards into each other (though the damage in this case is quite possibly more extensive).