Most likely all diseases branded "Ulcerative Colitis" do not share the same root cause. The commonality is the inflammation in the colon but the causation most likely consists of different subgroups. So, for example and just using random statistics for explanation, 25% could be caused by microbiome imbalance, 25% caused by true immune system malfunction, 25% caused by undetectable nanobacteria infection, 25% caused by very poor diet creating a breakdown of lining integrity.
One can convince his/herself that's the cause. Doesn't mean you are right though. Science doesn't have a definite answer on it, yet. So it's more of an opinion than anything else.
There's a lot of smart bugs, MAP is an example, that are very immune resistant. Everybody is exposed to them, but only a tiny fraction of us have an issue with them.
It's possible these smart bugs cause it, or are at least an opportunistic, secondary infection (I believe the later rather than the former). When the whole intestinal ecosystem is out of whack, it allows a lot of other baddies to gain a foothold and cause damage of their own.
Certainly, fascinating stuff from an intellectual level. I want answers too on causation. Hope they solve this within my lifetime, but am very pessimistic about that prospect.