When king Prithu performed 100 Asavmedhyagya , than due to jealously Indra stole his horse used in the yagya.
As mentioned in Srimad Bhagavat Puran canto 4, chapter 19 verse 23
यानि रूपाणि जगृहे इन्द्रो हयजिहीर्षयाः ।
तानि पापस्य खण्डानि लिंगं खण्डमिहोच्यते ।।
Indra with a desire to steal the horse , changed his appearance, and all those wrongful acts which are part ( खण्ड) of the sin ( पाप ), so they are called पाखण्ड