Of course the leadership could've arranged for the police force to retreat without informing them about the purpose. In that case, the rank-and-file officers are probably quite unhappy about that decision.
Or simply because protesters have made their lives very difficult for the past couple months and they're running out of patience and sympathy. Simple ingroup / outgroup dynamics.
But... As a counter argument, that could be exactly why law enforcement were pulled away this time. If they started cracking down on violent anti-protestors the peaceful protestors could have gotten caught in the mix and it could have escalated rapidly. I'm not defending Honk Kong leadership, but hopefully they're aware of the risk of concentrating armed forces amidst this tension.
If shots are fired it turns into a chain reaction.