what has happened to algernon? why is charlie worried?

June 21st- Algernon's behavior begins to change.....

Algernon, ....."appears to learn for the sake of solving the problem-success appears to be its own reward. But, as Burt pointed out at the convention, his behavior is erratic. Sometimes after, or even during a run, he will rage, throw himself against the walls of the maze, or curl up and refuse to work at all. Frustration? Or something deeper?"