We don't know yet because the police cannot yet show us the full footage from body cams and DB has only shown us an edited version from her phone. The police could have said more besides "other area" to the driver and DB, they could have given a list of reasons besides that, but all we have is an edited version AT
If the car's plates (the ones incorrectly entered) showed an issue (e.g outstanding warrants, offences etc), then I am sure the police would have mentioned that. They don't have to give any details, and seeing as how no ones knows the plates, there would be no data protection issues.
The reason they were pulled over seems to have been because the incorrectly entered plates did not match the car in front of them and this sparked 'curiosity'. Which it should do - as you said, cloned plates are a thing.
I would be very interested to see:
a) The full video from Dawn Butler
b) The body cam video from the police
c) The dash cam from the police car from the initial point they saw the car