You might think this is irrational. But keep in mind that they are sheep. Thinking for oneself is very costly for a sheep, so they adopt a decision rule of do as sheep do. Good sheep go with the flow. Why, if a sheep didn't go with the flow, the other sheep would smack right into it! You might think it is worse to be hit by a car than by other sheep, since they are so fluffy and all...but for a sheep, it is simply too costly to weigh those alternatives.