1. When she fell in the water sharks cropped up.
2. When Andre Agassi got outta the limo lots of girls cropped up screaming.
3. More drug users are cropping up day after day.
4. The match was abandoned as a fan cropped up on the field.The third one best fits the way the phrase is generally used. (Things appear as if from nowhere when the speaker is not around.) The others don't feel right to me.
The phrase "1 phrasal verbs" should be "one phrasal verb".~R