Because of the misalignment, one edge of the tooth extends beyond the end of the corresponding tooth on the corresponding gear. I would think the tooth that overlaps the other tooth would be slightly stronger at that location, due to lateral support of that part of the tooth extending beyond the other tooth that ends up short.
If you look at the picture,
The leading edge of third gear seems to be positioned behind the leading edge of the countershaft 3rd gear, and therefore at this location, the tooth on the countershaft would be alittle stonger than the tooth on third gear itself, and since the trailing edge of 3rd gear extends beyond the trailing edge of the courtershaft 3rd gear, the tooth on third gear is likely a little stronger than the tooth on the countershaft gear.
I think this may explain the failure pattern. Why the teeth on the front aspect of third gear failed, and the rear of the teeth on the countershaft gear failed.
Interesting.