In the first chain case, if you closely examine the edges where the chain link(s) broke... if part of the break edge is darker in color, or if the area looks smoother, it indicates that the chain had a partial break in this area. Newly snapped edges will be lighter (as dirt/oil will not worked it's way into the broken area like in any previously broken area). Also, in the area of the chain that may have broken first, the micro movement within the crack area works to knock down and smooth the exposed grain structure in the metal. My assumption would be that the chains doom probably first started with some micro fractures caused by some side over bending moment, possibly due to being jammed between chainring, or something similar. Then time, repeated loading, constant movement... all worked on it till it finally gave out when it was hit with a slightly greater load than normal. Once one area of a link broke, a chain reaction of breaking probably occurred. I'm guessing, as the pictures would need to be much more zoomed into the edges.