The bystander effect is most likely to occur because of

The bystander effect is most likely to occur when someone is witnessing an emergency and there are several other bystanders present.
If there is an accident happening somewhere, and nobody moves an inch to help, you are more likely to do whatever the rest of the people around you are doing (just stand and watch) rather than actually do something to help those people in need. This is known as the bystander effect.