Please see the image. The pH difference is maintained by a balance between the action of electron transport (increases the difference) and ATP synthesis (runs down the gradient/decreases the difference). With ATP production turned off by the poison, only the drive to increase the difference remains-- though there is probably an upper limit to this, as shown by the line eventually leveling off.
Work Step by Step
Ask yourself: why does the difference/gradient exist, and what generates it. What happens in the cell when there is a lack of ATP, and how does it relate to pH? Then consider what the poison mentioned in the question could do to disturb this balance