Mechanism behind chain wear phases
My suspicions is that the change relates to wearing through one material type into another, but it would be nice to get confirmation from people who know more.


Another hypothesis is that as the pin-to-link mating surfaces wear, they create an increasing gap for the entry of dirt.  Once dirt gets between the parts, wear accelerates due to a "wet sanding" action. At first smaller particles are able to enter, then larger ones as the clearance increases.

Eventual displacement of the fairly heavy-bodied factory lubricant might be a factor.

Exaggerated diagrams of the idea follow. Here is the state of a new chain: the pin is firmly pressed into the link with no clearance:



The arrows indicate the direction on of the stress under load. Then as the link and pin wear, there must appear a gap opposite to the where the load presses the parts together:



The "game changes" at this point. Of course the dirt works its way around the pin as the linkage rotates, and passes through sections of the drive cycle where it is not under load.