It’s a euphenism for spray-paint graffiti, trying to make them sound like it’s art of some sort. The way someone who does graffiti paints his name at the end of his work (like an artist would sign their painting) is referred to as his ‘tag’, hence the term ‘tagging’. Granted, occasionally people who are really good at it are paid to do installations on canvases, or hired to paint a mural on someone’s building, or tag a West Side Story set so it looks more authentic than it would if all the nerdy kids who built the set attempted to do it themselves, but by and large, ‘tagging’= vandalism.
Thanks, I got the impression that they were spray painting cars as they passed on the road.