Do police have to tell me why I'm being arrested?
Not necessarily. The police are not always required to tell you exactly what crime you're being accused of and arrested for. Usually, you have the right to be produced before a Judge within 72 hours of your arrest. At that time you will be arraigned, which means the charges will be read in public and an entry of a plea of not guilty will be done and bail will be addressed. Nonetheless, usually the police, at the time of your arrest, will typically give you some explanation about why you're being arrested. Always keep quiet when they tell you what they arrested you for so they don't use whatever you say against you later in court. This holds true no matter where you are (on the road or in a building or in the station house.) Request a lawyer as soon as possible thereafter.