Why would a pericardium be swollen?

Infection. The most common reason for pericardial thickening is infection. Viral, bacterial like tuberculosis etc. Rarely malignancy.
Not swollem inflamed. Many things can cause inflamation of pericardium or pericarditis. Virus like h influenza and coxsacki virus. Bacteria blike staph or strep. Autoimmimune dz like lupus. Connective tissue dz. Pts with renal failure can have uremic pericarditis. Post heart attack pts can have pericarditis as well. Just to mention a few.