What is the bullwhip effect and how does it relate to lack of coordination in a supply chain?

The bullwhip effect happens when retailers or other members of the supply chain overestimate a sudden increase in demand, and this causes a chain reaction in all the other participants of the supply chain that start requesting higher quantities of goods or materials for production. E.g. the fidget spinner was a very popular fad and its producers probably didn't anticipate how large the demand would be. Once the product became extremely popular, everyone wanted to sell fidget spinners. This caused an increase in the order quantities of all the supply chain. Once the fad faded out, all this momentum stopped and many stores, distributors, wholesalers, and even factories were left with huge unsold stocks of fidget spinners.

When the supply chain is well coordinated, there is little chance for some retailers or distributors to over react and want more product just in case. If your supply is guaranteed, then it would take some extraordinary increase in demand to make you want to increase your purchase orders. But if your supply chain is not well coordinated, you might fear that you will lose a lot of sales and other competitors will make them. Then you get anxious and start ordering large quantities.