Maybe you could compare the RSI grip warmer current load to the stock grip warmer current load. Or, ... Bottom Line: If the RSIs are warmer, they are using more current.
It is entirely possible that the load was too high. Especially, if you are running any other electrical add-ons. Most of the reading I have done, in the past, about availability of extra power on these sleds, say it is very limited. Thats why they had to mess with the grip warmers in the first place. When they started adding FI, they changed the grip warmer arrangement and that was when they started staying cold.
But, if the wire failed where it goes into the stator bundle, then it may very easily been the vibration that got it or a combination of vibration and current load.