LONDON — Iran is stepping up a crackdown against anti-government protests that pose the most audacious challenge to its clerical leadership since 2009. Following are factors that have brought Iranians to the streets and the challenges the authorities face.HOW SERIOUS ARE THE PROTESTS?The demonstrations, which began last week, are the most serious since unrest in 2009 that followed the disputed re-election of then-President Mahmoud Ahmadinejad.Political protests are rare in Iran, where security services are pervasive.