Why is this bus driver travelling on the road shoulder?

A viral video shows a bus driver travelling on the road shoulder.

The clip was posted on ROADS.sg and credited to Aaron Lee.

The video shows motorists travelling slowly on a packed road.

A bus can later be seen travelling on the road shoulder instead of in a proper lane. 

Road shoulders are not meant for drivers to use for overtaking other vehicles.

However, it is unclear why the bus driver used the road shoulder.