I've got soft, rotting wood above my shower or a bathroom window. Is this a plumbing issue?

Rotting wood, especially near a shower or window, is almost always a sign of chronic water intrusion. Check the caulking and grout around your shower or tub for cracks, or inspect the window flashing and sealant for failures. This isn't usually a plumbing pipe leak directly, but rather water escaping the enclosure or exterior envelope. Because it's a header, it's a structural component, so you'll need a licensed contractor to assess the damage and identify the source before replacement. This isn't a DIY job if the wood is truly rotten.