[Gaz64]

A few reasons for this to happen.

The coil lead has many times more sparks than each cylinder lead.
The coil end of the lead is hotter than the other end due to the heat of the coil.
The coil end boot could have had a compromised boot allowing moisture in.
The male to female connection might not have been as tight as could be.
An finally, the dissimilar metals involved, combined with any or all of the above, will give various levels of this phenomenon.

I have had cases where the lead has corroded so badly, that the bakelite tower has cracked from the growth buildup.

Gary