Which immunosuppression caused by long-term usage of these antibiotics in untreated celiac disease?
One possibility is that these antibiotics could disrupt your normal gut bacteria. Given that your gut has the surface area of a tennis court and is typically coated with symbiotic microbes, antimicrobial therapies could dramatically change the population dynamics of your gut ecosystem. One might imagine that the immune system of the gut is in homeostasis with these gut microbes, and if they all die off all at once because you're taking some sort of 'cillin, your immune system might suffer for lack of stimulation. Fewer immune cytokines released by your intestinal immune cells into your blood => systemic reduction in immune function.

Other possibilities could include: the drug just randomly fits into a bad spot on some important immune protein; the drug targets bacterial-related genes in the mitochondrial genome and disrupts the metabolism of the immune cells; the drug disrupts your ability to absorb nutrients required for immune function...the list of potential mechanisms is undoubtedly long.