What can cause a never-frozen indoor copper drain pipe to crack?
I would inspect the entire property for more such pipe and replace any that was found.  That's almost certainly a manufacturing defect and likely to be pending failure wherever that batch of pipe was used.

That is not a natural crack, nor a corrosion crack, it's too straight and uniform.

Typical "natural" cracks look like:

&emsp; &emsp;

Possible ways it could have formed:


There was a defect/contamination in the extrusion die for that batch of pipe, propagating weakness down the length of pipe as it was extruded.
The pipe was rolled rather than extruded and then poorly joined (trying to make it look seamless).
Something/someone else uniformly scored the length of the pipe before it was installed.


Then after some years of thermal cycles and possibly mild corrosion, a crack formed and rapidly propagated along the rest of the weak line.