I'll bet the Troopers had advanced evidence and knew the guy was carrying but their evidence was either inadmissible or they wanted to keep it secret due to other ongoing investigations. So they tagged a canine unit behind the guy and just waited for him to make the slightest traffic violation. Bam - busted.
So it's ok because they might have already illegally searched his house?
It's possible, sure. Wouldn't be the first time, though.