This can best be explained in terms of the power of suggestion and the tendency of the human brain to try to make sense of incomplete information. When people are presented with ambiguous or incomplete information, their brains will often fill in the gaps with assumptions or prior knowledge. In this case, some members of the class thought they saw a giraffe in the car-crash video because they were primed to look for something unusual or unexpected. Once someone suggested that there might be a giraffe in the video, others may have looked for evidence to support this idea, leading to a confirmation bias. However, it is important to note that this is just a hypothesis and there may be other explanations as well. (Not sure if this is what you meant.)