Has anyone seen one of these connecting rods break before? Just trying to figure out what would have caused it. Snapped while riding up a smooth, steep slab. No case or large compression, so it's really strange. Just wondering what the root cause might have been to hopefully prevent it from happening again when I replace the part. Any ideas or suggestions would be greatly appreciated! 

Wow. thats odd indeed.  i see no hits from rocks on the knuckle, my uneducated guess  ?   bad metal.   or-perhaps the bolts were over tightened ?  

Were all the spacers / dust shields etc, present when you did assembled it ? If any were missing this could easily be the culprit.

Only thing I can think of is that it hasn't been assembled correctly (spacer missing or on wrong side etc) making the whole system run out of alignment causing it to twist under compression.

Either that or it was just a faulty part, but you would expect to be able to see some form of imperfection along the break line to indicate this.