It’s physics. My understanding is that sound makes waves that we cannot see. Each tone has its own frequency. So when you play C, the string vibrates and with its wave pulls down other waves at the same frequency – i.e. all C strings within range of the wave will sound, even on another ukulele.
I was finding out because something similar happened to me.
I played a G string and heard the same tone from the opposite corner of the room. Mystery! I repeated it and followed the sound. A child’s guitar packed in a cardboard box rang out. 🙂