AirDrop is very useful for transferring files between iPhones, but if the settings are wrong, you could find strangers sending you photos you would rather not see.
It happens when the iPhone’s AirDrop settings are set to allow anyone to send you files via AirDrop. It is possible to set it to contacts only, but if you use the feature a lot, you may have set it so everyone can send files. It is a pain to turn off contacts-only/everyone, so people often leave it at everyone.
In public places, especially where there are a lot of people, someone can send you photos using AirDrop that you really don’t want. If there are plenty of people around, you won’t know who it is, but you’ll have photos you didn’t want on your screen.