Judge Berzon concurs in today's opinion and says that, although it's not argued here, maybe she'd find a right under the Due Process Clause to be informed of the true reason for your arrest, rather than be told a made-up story by the police about why you were busted. (Here, after some wiretaps, the police stopped a drug-laden vehicle, but told the driver that he was being stopped for an illegal lane change -- a lie -- because the police did not want to reveal the nature of the ongoing investigation.)
Judge Berzon argues that several states, including California, have adopted precisely such a rule as a matter of good practice. And California does indeed have such a rule.
But I'll nonetheless mention that this rule contains important exceptions, including one that'd be applicable here. In California, yes, you have a right to be told why you're being arrested. But not if the officer has reasonable cause to believe that the arrestee is -- as here -- actually engaged in the commission of an offense. (Ditto for if he's arrested immediately after committing the offense or after an escape.)
So don't be thinking that, in California, you always necessarily get told why you're being arrested. In many cases, there's indeed no such state-created right.