poor grounding is usually the bane of older engines as the lug eventually gets enough corrosion between it and the frame. For the M3W it is the issue of vibration. Probably the bolt was not tight enough to start, but I had to replace an exhaust shield bolt within the first 6 months of owning mine... just vibrated loose! Did you ever notice NOTHING every vibrates tight!

Rodger
2018 M3W "Dreamers on the Rise"
2004 Plus 8 (wife's)
old Goldwings and a couple of Harleys