Three lines = grade 5 bolt.
Six lines = grade 8 bolt.
Tensile strength of a grade 5 bolt is 85,000 PSI. To fail that this bolt in tension requires 6800 LB of force. To fail this bolt in torsion, as in over-tightening, requires only 47 FT-LB. Assume we torque the bolt to only 75% of this value for safety's sake to 35 FT-LB.
For a grade 8 bolt, the tensile strength is 120,000 PSI. Therefore, it will fail in tension with 9600 Lf of force or 66 FT-LB or torque.
To put into perspective, the oil pan plug is tightened to 33 FT-LB and that is a larger bolt. Another reality data point is the wheel lug nuts......slightly larger threads torqued to 80 FT-LB.
Assuming the grade 5 nor grade 8 bolt was over-torqued, the failure mode may be from fatigue, which is somewhat representative of the picture, but I'm not 100% sure because of my browser's image resolution. As Arata mentions, a fastener that is repeatedly used, will over time it fatigue fail. However, this bolt is a one-time application so the fatigue source would be more difficult to trace.
As far as using a larger bolt, the next largest is 5/16". The female hole in the end of the shaft has to be drilled out. If the remaining 3/8" threads are still present even a bit, then you have to go to the next largest bolt which is 1/2" Relative to the shaft diameter, that is huge and may invite higher shaft stress in the bolt area.