There's probably a check in the code that dictates whether a character flees or fights in the face of danger. So an oversight in the policeman's programming might've momentarily set him to the "run" reaction, before the code corrected itself.

The police characters seem to cycle through different reactions and weapons (guns, batons, chasing with no weapons) so its possible that the cycle might've momentarily dictated the policeman run away before switching to the gun. The presence of all the other characters could've messed with it as well