The bullwhip effect can be described as an extreme change in the supply chain upstream that is normally triggered by a small change in demand downstream. This often affects inventory levels which can shift from being on backorder to being in excess of the needed demand. The general cause of these fluctuations within the supply chain is almost always related to communication. As communication travels up the supply chain with various forms of waste and delay amounts, times and the accuracy tend to "whip" further from the actual need. The one sure fire way to eliminate the bullwhip effect is to align supply with demand, including completely transparent communication and synchronization of the supply chain perfectly.
Reference: Apics Dictionary 2015 - Bullwhip effect.