Part of the reason for my Mum's increase was given as being involved in a collision. When we drilled into the reason, it turned out to be when my Mum was driving down a road and someone reversed out of a driveway straight into the side of her.
Mum's car was written off, the woman lied through her teeth about what happened, but eventually after months of wrangling, ending with the 3rd party being threatened with court by Mum's then insco, she admitted liability and her insurance coughed up for all the losses.
When I asked Mum's new insco why this accident put their, premium up when a full recovery was made and indeed her insco at the time did not increase any premiums at renewal after the accident, they said that because she had had the accident, that was enough to increase the risk, regardless of blame!