[praptak]

Why is it called bullwhip effect?

[Phillipharryt]

A small movement at the handle of a bullwhip produces an enormous (and oscillating) movement further down the the whip, it's an analogy for movements in customer demand having large effects further down the supply chain.

[bmn__]

See first illustration: http://enwp.org/Bullwhip_effect The graph representing over-/undersupply resembles the movements of a whip.

Out of curiosity: did you try to look up the answer to your question before asking it here?