Your boiler is simply performing a routine cooling-down process after you have finished using hot water, so there is no need for concern.

DHWmode:Pumpoverrun

What causes this fault?

This fault occurs when the boiler's sensors detect that water is returning to the unit too quickly without losing heat, which usually means the water isn't circulating through your radiators properly. The most common reason is that the valves on your radiators are closed or there is a physical blockage in the pipework preventing the flow. It can also be triggered if the boiler's internal pump is struggling to push water around the system.