Ming Chun Tang in the alternative media CounterPunch says that the protests are against the neoliberal economic policies that are destroying the prospects of everyone but the one percent. In other words, the protests are akin to the American occupy movement.
Another explanation is that once again, as in Kiev, gullible westernized students have been organized by the CIA and US-financed NGOs to take to the streets in hopes that the protests will spread from Hong Kong to other Chinese cities. The Chinese, like the Russians, have been extremely careless in permitting Washington to operate within their countries and to develop fifth columns.
Americans are forever deceived. Remember the bullshit about "Mission Accomplished!"
The only mission that has been accomplished is the enrichment of the military/security complex and the creation of the American Police State.