The reaction mechanism is that of the related Hofmann degradation. 
At first the carbonic acid amide (1) reacts with the sodium hypochlorite. After the separation of water and chloride an amine with a free bond is built 2. The intermediate (3) is generated by rearrangement. In the next step a hydrolysis takes place. Water is added at the carbon-atom with the number 1. A hydroxylic group is generated. The last step is that an acidic amide is separated and the aldehyde (4) is generated.