The reason is that : Some people use this phrase in a wrong manner.
If you say: I could not attend the party yesterday.The reason is because I was busy, it is considered as redundant. Redundant means repeating the same word twice.
In the above sentence reason itself is similar to because. So, never say' the reason is because'.
The correct way is: I could not attend the party yesterday. The reason is that I was busy.
Raj resigned from the job. The reason is that he was not satisfied with the salary he was getting.
The company will not function from next month. The reason is that it has undergone a heavy loss.