What is the reason of non-deterministic (indeterminate) behavior of concurrent programs?

Concurrent programs are the programs that execute at the same point of time. They are simultaneous in nature with other concurrent programs.They are executed with the help of threads  to achieve the concurrency without the issue of scheduling. They are consider long running programs.

The program that has the low execution time compared with all other programs gets executed first.If the case of no progress is seen then another thread is executed.This type of execution gives the non- deterministic situation in which the possibility of leading completion of any particular program cannot be determined.