Multiple answers were allowed here. Such a maximum is the number after which adding more processors doesn't reduce the application's execution time. This number represents a trade-off. Beyond it, additional processors don't reduce the application's execution time enough to make it worth requesting them. Low communication means that the processes run almost independently.
Heavy communication has noticeable impact on the performance of the application.
It can be due to a bug, an invalid input, a system shutdown, or anything else.