Visible crew/equipment: In the scene in the beginning where Will Smith chases the alien who jumps off the bridge onto the street. Will Smith jumps from the bridge onto a tour bus, then jumps on the back of a newspaper delivery van. In the passenger side of the van is a cameraman with his back towards the windshield in order to film down the side of the van as Will Smith jumps off the van. (00:10:20)
Visible crew/equipment: When J puts the glasses on for the first time, a lighting screen is reflected on the right lens. (00:38:30)
Visible crew/equipment: When Tommy Lee Jones shakes the dog asking him about the galaxy, right before they exit the car, you can see the cameraman's reflection in the shiny black door frame. (01:06:30)
Visible crew/equipment: When J is going to use the neuralyzer on K, on J's lenses you can notice that K has been replaced with a white screen used for lighting.
Visible crew/equipment: As K goes to tell Mikey to be quiet, the corner of the sound stage can be seen over his left shoulder.
Answer: K and his partner both knew about aliens living on Earth, and made it their duty to make sure no one else knew about it. When the trooper saw Mikey in his real form, and Mikey spotted him, he attacked the trooper in an attempt to kill him, probably to ensure that the trooper wouldn't be alive to expose them. Since K and the other agent were already aware of Mikey, as they had dealings with him before, Mikey didn't attack them.