Your boiler is briefly keeping the internal pump and fan running to clear away excess heat after you have finished using hot water.

What causes this fault?

This occurs because the internal components of the boiler have become very hot while heating your water. The boiler is simply running the pump and fan for a short period to circulate water and air, which safely carries away that residual heat and protects the internal parts from damage. This is a normal part of the boiler's operation and not a sign of a mechanical failure.