Why are yawns contagious?

Contagion
Con tangere
Together with
the touch
Against
The opposite and adjacent
Contagious
Communicable
Spread of things
Good or bad
such as laughter or merriment
or yawning
Contagiousness
To spread and affect
With or without contact
Infectious
Transmission
Through the environment
Influence
Extend

I watched a yawn wave crash over the audience of poets and prose writers today.

Yawn
To open one’s mouth
Gaping
To open one’s mouth
Wide open
Deep Huge
Great!
Reflexive act
of opening one’s mouth
Action
Reaction
To open one’s mouth
and inhale
To open one’s mouth
Involuntary
Inherently empathetic
and aware

Said to be an action that circulates
Through nearby bystanders
Who want to understand the human condition
And therefore, subconsciously, try out the act.