Most people have seen examples of Contagious Yawning in humans. The causes may be for instance, tiredness or boredom - and if one person in a group yawns (or convincingly feigns a yawn) it's often quite hard for others to resist.
Contagious yawing has also been observed and scientifically documented in many other animals including :
There are theories regarding the possible evolutionary advantages to yawn contagion, for example, that contagious yawning may be part of a neural network involved in empathy. reference
But a full explanation is currently lacking.
Also see: Yawning