Just speculating so factor that.

With the understanding that modern chains have plates thinned to the point of having minimal safety margin, we can expect greater instances of breakage owing to variables.

There's always a tolerance in manufacturing precision, alloy formula, heat treating, etc, so there's always a weakest link. With little margin for error, what used to be OK no longer is.

However, I'm not convinced that the failure near the master link is coincidence. 66:1 odds (either side of connector in 122 link chain) argue against that.

So consider that each inner plate carries half the load roughly equally, and can't carry much more.

The dimensional precision of the connector is more complicated than the normal plates, so it's conceivable that one side is a bit off. That would alter the 50/50 load sharing of the adjacent inner plates, taking the harder working one above the fatigue limit, and eventually to failure.
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BTW I noticed some nicking of the outer plate in the photo. This is usually caused by shifting under load. Not that it caused the break, but it is hard on chains and not a good thing. If you see much of this type of damage, take a hint and try to time your shifts better.