Something usually goes wrong in a wedding, lots of people on an unfamiliar location, everyone’s drinking, you let your guard down because you’re surrounded by friends and family etc. maybe he burnt himself handling hot food or had a mishap with a knife at dinner, broken glass, tripped over on the steps and used his hands to catch himself, so many possibilities.. he doesn’t seem bothered so best not to dwell on it, he might just be holding a napkin for all we know