Ricardo,

This effect is temperature related. If you consider the longitudinal axis of vessel splits the vessel into two; in case one half gets cooler/hotter than the other half the vessel will bow due to the temperature difference.

You can see this effect mostly on the pipelines that get hotter on one side under the sunlight.

The same thing can occur in the process vessels if heating/cooling is not uniform.

Hope it helps.

Ibrahim Demir