Good question. The natural way to say it is actually "reason enough", and to my ear at least, "enough reason" sounds wrong. It's true that in English, the adjective usually precedes the noun, but in this case, I think the word "reason" must be working more like an adjective, and "enough" more as an adverb qualifying it, along the lines of other expressions like "good enough" or "fast enough". However, note that when "reasons" is plural and is clearly working like a normal noun, then you would generally say "I'd had enough reasons" (although even then, "reasons enough" is acceptable, again to my ear at least).

That's exactly what I'm confused with. 'Reason' is a noun, isn't it? I think you could be helped with the answer given by James. Good luck!

It's "enough reason". When you use "enough" to describe adjectives, you should put "enough" behind the adjectives, such as "good enough, big enough". Hope that this can help you.