Why did they arrest one person at this college party?

i went the wrong direction and turned down a popular street filled frats. i have been to many frat parties but dont go to these ones often. i saw millions of people on the street drunk and stuff. but infront of me the whole way their were cops. and they turned down the same street randomly they stopped and some kid who was walking with a few girls started to run. the cops tackled him and arrested him. without looking the girls jumped into the car and drove away as if they could care less. was mexican. millions of other students were walking around the cops drunk not getting arrested. why is this. why did they know at that time to go down the street arrest that specific kid not the girls. do girls call the cops when they are at parties and get people in trouble or did the cops just pick out one person and that was it? i have been to millions of frats but i go to the smaller ones off campus. I have never seen a cop in my life their. What could he have done?