My current state of thought:
The lean seems likely to be more strongly linked to proprioception than to visual or vestibular feedback.
My vague memory is that riders mostly keep their heads level, even when their bodies are leaning a long way. Photos would help, replacing unreliable memory. I think afflicted individuals lean in a consistent direction in multiple events but I don’t know many people who experienced the lean on multiple events. The lean doesn’t relate to road camber but is almost always experienced by older riders.
https://en.m.wikipedia.org/wiki/Proprioception gives a thumbnail explanation of how muscle length and pressure is automatically assessed, together with limb movement approaching joint angle limits. Riders’ perception of feeling as though they are sitting upright on their bike, despite actually experiencing a significant lean, seems to align with a mismatch between actual and perceived muscle feedback.
Steve Hogg, who’s opinions regarding bike fit approaches a significant level of woo, thinks that proprioception and body asymmetry significantly affects how people sit on bicycles.
https://www.bicycling.com/news/a20036285/steve-hogg/I agree with that basic concept, if not a number of the factors that he thinks are influences.
Riders tend to need adjustments in their bike fit to optimise comfort and power as their flexibility, training and strength alters over time i.e. bike fit is a moving target. Older riders tend to lose muscle, height, flexibility and power compared to their youth but are often resistant to altering a riding position that “has worked well for me for years”.
Long events are all about remaining comfortable and maintaining consistent power.
Perhaps the lean is caused by fatigue of secondary muscles that have been overloaded during long bike rides by poor core strength and imperfect bike fit?
Time for folk to poke holes in my ramblings and for somebody who actually knows about this subject to set me straight or suggest ways to take this further.
