I have not really researched too much about causes to be able to declare it is this that or the other, but like you I am a believer that it is connected to our digestive system, because my two friends who got psoriasis like me when we were younger life was easier, food was cleaner and it was not as toxic.
My friends like me became under lots of stress, we ate one too many processed food ( or else we thought we were eating good but we were in fact not as meat is no longer meat, fruits are covered in chemicals etc) and then i got psoriasis, my friend got psoriasis and solved hers by eating fish and green veggies, and another friend of mine and her family are all breaking out in skin conditions.
We are obviously being poisoned by our bodies and environments, it is re-addressing the balance.
I think a big part with me is I do things excessive, like having yoghurts every day until I developed an allergy, eating fish every day until I developed nausea, if I like something, it becomes my flavour of the month and I will eat it until i am sick of it, dates, fruits etc, so I think that is my weakness and one I need to address, because previously to my bad flare up I was having jacket potato more or less every other day etc, variety and keeping it clean and simple is the key, not to mention minimal amount of stress and some exercises.