There may be a reason even if they didn’t tell you.
The burden of proof to stop someone is called “reasonable suspicion”. The burden of proof to search someone is called “probable cause”.
When it appears that you have been stopped for no reason it is called a “Terry Stop”. The name comes from the standards established in a 1968 case, Terry v. Ohio. Remember, all the police need at this point is a ‘reasonable suspicion.’ Police will use a Terry Stop as a tool if he or she observes unusual conduct which leads him or her to reasonably suspect that criminal activity may be occurring. This is a gray area and left to interpretation.
In the case of a moving vehicle, a stop can be made even with the most minimal of infraction or safety issue. The police have the right to make inquires and they will ask if it is OK to search your vehicle. This is where your fourth amendment rights come in. If you don’t care if the police search you or your vehicle then you can give them permission. In some cases this may be a signed waiver.
If you say NO. Police can detain you for a “Canine Sniff” to get “Probable Cause“. If the dog gets excited, you will be arrested and the vehicle will be towed and impounded for an inventory search.
Calling in the K-9 unit is another stall tactic and of course procedure. While you are waiting you could ask: “Officer, am I free to go?” You have the right to make this request at any time during the stop. The office of course will say no. You then have the right to remain silent and not give consent to any searches.
Police don’t always need a warrant to search a vehicle. Your personal property, be it you or your vehicle, is on public property (the roads) and the warrant rules are loose if existent at all. It may depend on your state, but for a police officer to search your vehicle without a warrant they would need ‘Probable Cause’. This means they are arresting you and they have reasonable belief that you have committed a crime.
Yes, a police officer can stop you for no reason.
If you feel that your rights are being violated try to record the incident with your phone or call out for witnesses. This will help keep the police officers in check however, there is a fine line here where a harmless stop could escalate into an arrest. Keep your cool around police officers. Remember they are the ones with the guns.