I know why! This is classic and ok. My mother "lost" one every year for a long time and then gave up on them. They have a summer dormancy cycle! Yours is happily doing what it ts supposed to do. Let it get a bit drier than you usually do, don't change it's lighting, and sometime around mid August pick up with the water again. Some varieties loose all their leaves, some just most, some just get really ratty looking, but it'll come back, maybe with
flowers before leaves.