CC happens when impact is made within a very small window of time during the cardiac cycle. It's very rare because the window is so small. Just comes down to bad luck pretty much.
I've read a bunch about this and even though its very rare because of the exactness of the timing of the hit needed, there are thousands of these collisions in an NFL game. How has this not happened before? Are there really no underlying conditions with an event like this? NFL hasn't ever seen anything like this across thousands of games, millions of hits and tackles, etc. How?