Why would undercover cops pull me over for a traffic stop? I thought they work with narcotics.?

I was pulled over by a regular looking SUV and two men in jeans, sweatshirts and a mustashe and beard pulled me over for supposedly running 2 stop signs, having no functioning tail lights and no seat belt. This blew their cover, so why did they do that? Why would I need to be stopped when it's risking their cover as undercover policemen?