It's a bot—it seems to tweet six times an hour, every ten minutes.
/u/burningempires has shown that it may be using a fixed corpus for its source material, since they have found it previously tweeted similar casualty figures.
/u/K-Vox wondered about the algorithm used. /u/FIleCorrupted mentioned Markov Chains, which I believe is correct.
It's running an algorithm very much like Dissociated press, which is a very old concept. I wrote one years ago. The sometimes ungrammatical output of @PZbooks matches exactly what I used to see in my implementation years ago.
Basically, it's an old Computer Science practical joke that got lucky via the sheer volume of attempts. As /u/K-Vox said:
…just a cyber monkey on a typewriter.