PCB Warpage Issue in PCB manufacturing
The problem you mentioned is due to imbalance in thermal expand/shrink between layers (in this case copper vs base material, like laminate). When the copper from one side of PCB is completely etched it tends to warp when the copper on the other side cools. It happens when there is non-even number of layers.

Solution? Use double sided PCB (or any even number of layers) or instead of solid copper try to use grid copper (this is named "hatch" in PCB CAD program). 

Please see https://www.multi-circuit-boards.eu/en/pcb-design-aid/copper-balance.html