Humans are pack animals. The instructors were likely seen as not his group. What the AAIB cannot investigate is his flying social group, where this sort of flying was discussed.
A C150 Aerobat joining with 2 Maules on farmstriping is a labrador running off to join the wolf pack.
The reluctance to wear shoulder harness is weird. Otherwise he was just moving into a new flying scenario, which was possibly unfamiliar to the instructors he knew.