All modern bullets have a tendency to "tumble" (yaw) when impacting something that will deflect them. The .303 British (which is very similar to the 30-06) had more yaw tendency because its lead base was relatively heavier than most comparable bullets. E.g., the difference in handling of a rear engine - rear drive vehicle vice a front engine - front drive vehicle. Basic Wiki stuff: Ballistic trauma and Yaw (rotation).

Regards

Mike