The leans can happen to anyone and by that I mean those without underlying vestibular conditions.
It can happen in VMC when your visual reference of the horizon is removed or altered. An example of this is a cloud layer that isn't horizontal but obscures the actual horizon and can cause the pilot to adopt a 'wing down' attitude.
In mountains, where you have high ground all around and rock strata lying at odd angles, the illusion can be powerful enough that when you see something that is actually horizontal, the brain struggles to compute - there is a lake in Snowdonia known as the sloping lake for this reason, you fly up a valley with several turns in it and then come round the corner to see what you thought was level actually isn't. We always taught that mountain flying is a visual/instrument balance to try to keep orientation.
In the Bryant crash you have two of the possible illusions that I have described - low lying cloud, probably in layers, and hilly terrain - the combination is dangerous to start with but if you then enter cloud and are not practised on instruments, it is no surprise that disorientation occurs.
If you remove the brain's horizon reference (or alter it ) and don't replace it with a learned procedure - transferring to instruments and believing them - then things are going to get tricky very quickly.