How does this even happen?
I tell all my non-cycling friends that I have that much power!
Probably a combination of fatigue in the metal and that link having been knocked and damaged at some point. I’ve had a chain on an MTB where it’s happened on three links (not at the same time!). In my case the chain was three years old and had done nearly 4000km, there was nearly as much value in the quicklinks I used to repair things as the rest of the chain.
A 1x drivetrain by any chance? The chain undergoes more lateral flexing on such systems, as such the chain is laterally flexible to a small amount to compensate for this but the metal is still being stressed in its weakest axis.
That looks to me like something was told to fit where it wouldn’t, likely between a chainring tooth and the two outer plates.
In other words, a piece of something was jammed between the ring and chain and the plate gave out before the debris.
Chain defect perhaps? It happened to my cleaned and new (less than 100miles) KMC chain on road up a little incline where I certainly wasn’t putting out too much power (at a guess 250w, I typically only put out 700w in a full out sprint).
Edit: looking at the pictures my chain although warped held together
some chains/brands/types/batches/… start showing cracks at some point, not sure what the cause is, but I think it’s just bad luck (bad batch).
check these images:
often there is not just 1 crack, but many, if it’s fairly new, you might try to claim warranty.
Do you have power on that bike? (or any), if yes, whats your peak power?
Chain was factory, 4 years old. I guess it was time to go. Put a lot of km on it.