The only thing that occurs to me is that he may have seen the ground and gone for a gap.
We know that low level stratus/hill fog is not consistent in its height or thickness but when you combine upslope stratus with slopes, a glimpse of the ground may give you a completely false horizon and the brain can easily be fooled into believing you are wings level rather than in a turn.
Visual cues are very difficult to overcome - as anyone who has had the leans from going in and out of cloud will tell you.
We used to show pilots a lake in Snowdonia which, in the right conditions, looked like it was sloping (it was referred to as the sloping lake) all it needed was a lowish cloudbase to obscure the tops of the hills and the steep sides of the narrow valley did the rest - illusion complete and difficult to fight unless you believe your instruments, something that is not easy to force yourself to do at low level.. That was a reason we always taught mountain flying as a visual/instrument balance.