Do dairy, gluten, or processed foods trigger IBD?
For most people, dairy and gluten do not cause or worsen the inflammation of IBD. They can still cause gut symptoms in some people, for example the lactose in milk or the fibres in wheat, but that is about tolerance, not inflammation. Highly processed foods are a little different: cutting back on them fits general healthy-eating advice and may help. The key is to test your own tolerance with a dietitian rather than cut whole food groups on assumption.
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These three get blamed a lot, so it helps to separate what causes inflammation from what causes symptoms. They are not the same thing.
Dairy and gluten: symptoms, not inflammation
The evidence does not support the idea that dairy or gluten drive IBD inflammation. A systematic review of milk and dairy concluded there is no clear evidence they influence the course of IBD, and that studies do not confirm dairy is linked to disease flares (dairy systematic review). The same review notes that dairy is one of the most commonly cut-out foods, which can backfire by leaving people short on calcium (dairy systematic review).
What can happen is a symptom reaction without inflammation. The Crohn's & Colitis Foundation explains that there is no evidence gluten or dairy cause or worsen IBD inflammation, but in some people the lactose in milk or the fibres in wheat can still upset the gut (Crohn's & Colitis Foundation). That is about your own tolerance, not the disease itself.
Processed foods
Highly processed foods are the one part of this question where cutting back has more support. The Foundation notes that limiting them, in line with general healthy-eating advice, may be beneficial (Crohn's & Colitis Foundation).
Test your tolerance, do not assume
The practical takeaway is to find out how you personally react, ideally with a dietitian, rather than dropping milk, wheat, or other staples on a guess and risking a less balanced diet.