Travelling in a bumpy bus

Why one needs to keep ones legs far apart when he has to stand in the aisle of a bumpy riding bus?

 

[HallsofIvy]

In order to stay upright, your center of mass has to stay above your base. Placing your feet farther apart makes your base- the area between your feet- larger. The reason you need to do this on a bumpy bus but not when you are standing on the ground is that your center of mass moves more on the bus.