He might have caught his fins on something or they might have torn if you use a net to get him out of the tank during cleaning. Do you have any fake plants inside of your tank that are plastic, or have silk plants with plastic stems? These decorations can snag on your betta's fins when he decides to cruise through them.
Keep his water clean and remove any plants that don't pass the "snag" test - a.k.a. if you take the plant and run it along a pair of panty hose, if it snags, it could tear your betta's delicate fins.