Asparagus contains a type of poorly absorbed oligosaccharides called fructans. These short chain carbohydrates are poorly absorbed in everyone'es gut however people with IBD (UC included) suffer from exagerated symptoms from them because their gut is already compromised.
Frustans INCREASE the permeability of the gut lining and increase the irritation therefore. IF they do this in a normal gut imagine what they'll do to a sensitive one.
This is why i follow the low FODMAPS diet. Strictly now. Im a nutritionist btw.