Well, many students often confuse the two combinations. There is a HUGE difference in their meaning.
Behind, as the word suggests, is BEHIND something. It’s HIDDEN. You can’t see it clearly. Rather, it is INVISIBLE. So, when you write “reason behind something”, the reason is hidden or it may have been hidden in the recent past.
We are not aware of the reasons behind this catastrophe.
I will have to apply for viewing my answer sheet to determine the reason behind the low score.
“Reason for” stands simply for WHY something happened. You KNOW the reasons. You can clearly SEE them.
The reason for my low score is the lack of preparation for geometry and calculus.