I assume it was weak soldering around the plugs, I never actually found out the cause. For both sets that I had they lost sound in the right ear cup within a weak. And these were supposedly the refreshed design that was supposed to address that issue.

One set was bought through amazon, the other through apos.

You'll notice that there is no protection where the cable plugs into the headphone meaning any little bump or nudge can stress the plug in. This compounds the issue with weak soldering for the connections. Other headphones have a recessed plug that helps protect from this issue.