Probably old leaves getting ready to make room for new ones. You want to remove old, dying leaves anyway to keep down potential food for fungus and mildew.

Watch for mildew and spray for it if you find it. The stem is what you want to keep an eye open for because if it rots or dries up, then more than likely (not always) you will lose the entire plant. Give it lots of light, water when somewhat dry, and fertilizer when it is actively growing again.