Age I would imagine. Over time with all the heating, expanding, contracting solder joints can come under strain. Also component value changes and degrade...bit like us humans really. We start off new and firing on all cylinders and it's downhill from there
Generally speaking though Yamaha gear is built to last and it often does so things like this are an exception. Looking back at all the keyboards I have owned which is around 31 over the last 30 years I have only needed to open one for repair and that was a Roland D-50 that had a dodgy resistor. Took about an hour to fix including disassembly and reassembly. So I think that is pretty good going and shows the general reliability of keyboards from Yamaha, Roland, Korg, Emu, Ensoniq, Alesis etc.