Football Violence: Why?

The news from Egypt today is very distressing. There has been a horrible riot at a football match in Port Said in which hundreds were injured, and over 70 killed.
Outbreaks of football related violence have been linked to the absence of police on the streets in the post-revolutionary period.
From a psychological standpoint, I wonder why fans are turning to violence around football in this period? Is this an outlet of frustrations on other fronts?
 I just learned the terrible news that an American University in Cairo student was among those killed. President Lisa Anderson issued this statement. 
Dear AUC Community,
I am deeply saddened to report that one of our students, Omar Aly Saad Mohsen, a senior scheduled to graduate this February in economics, was among the 79 people killed in the tragic soccer match that took place yesterday in Port Said.
To mourn the loss of our student and the many other victims who lost their lives, the University will observe a day of mourning on Sunday, February 5. The University will be open but all classes are suspended and a University-wide memorial service will be held at noon in Bartlett Plaza.
We extend our condolences to Omar’s family and friends and all others who feel this terrible loss.
-- Lisa Anderson President The American University in Cairo