What was this police officer trying to do?

So I am driving at 2 am down a major street. I am doing 70 km/h in a 60 zone. Which is normal, even though it's speeding. The car behind me is doing the same. I am on the left lane, another car is on the right lane. I slowly go to overtake, then the car ahead turns his flashers on. Turns out, it's an unmarked car. He slows down...I slow too well...to about 50 km/h, that was still more than enough to overtake. He look back...he's stopped. 5 seconds later, he turns off his flashers, and makes a right turn. Was he trying to tell me to slow down...or was it unrelated?