I can't prove anything, but I've noticed sometimes that my UC is worse when I have a cold. One of my big triggers is stress, so I wonder if that is part of it. I also wonder if my immune system ramping up to fight the cold also is part of it.
I've also noticed that when my allergies are bad, and I have a lot of post-nasal drip, that I think all that stuff going down my throat upsets my GI tract (like right at the moment - the ragweed is bad today).