Those are random grenade throws. When you play enough you'll notice patterns people run. On CoD experienced players can toss grenades right at the beginning of a match knowing that inexperienced players will just be blindly running towards the middle or to a certain area of the map. An easy kill or multi-kill if you know where to throw them. Once again something that you'll learn as you play is that there are choke points that if you're lucky you can catch a group as they run towards an objective but it's down to luck. I myself would throw grenades at the doorways on Moon as a enemy got closer to try to time it to hit as they entered the doorways sometimes it'd work sometimes it wouldn't.