It is likely to be an alleged non-payment of a fine or something like that, even if only a small amount remains outstanding. When a warrant is issued by the court, the police (or usually one of their employees) have no choice in whether to arrest the person, but they are usually released on bail immediately. Some years ago, when I was a police officer. I had to bring someone back from Scotland to the Midlands when he was accused of driving without insurance and having no water in his washer bottle. In that case, he had failed to turn up at court owing to a misunderstanding. If he was wanted for something serious, he would be in custody by now.