Other than being palmed off with a lemon car, I suspect that most people ditch cars because they:

1. Are too expensive to run. Sadly all too common these days, with people naively thinking that buying a £10k high performance sports saloon or uber-luxury car that originally cost 5-15x as much new will cost buttons (i.e. as much as a Mondeo) to run.

A car that cost £50k new will cost a good bit more in fuel (because its likely larger, heavier and more powerful), insure (more expensive parts, more gizmos that could/will go wrong [higher price certainly doesn't guarantee better reliability, quite often the opposite these days]), tax and service/maintain.

2. Previous owners could've neglected the car either by not driving it as designed and/or maintaining it on the cheap.

Once it's gone past 3 owners, I would start to get very wary of this unless it had a fully (proven) documented maintenance history as per the manufacurer's guidelines and you could somehow contact (e.g. via specialist owners' clubs) all the previous owners to judge for yourself how they treated the car.

3. As you say, people will often get bored, especially if the cost of ownership is high and the lack of seat / ride comfort or lack of suitability for longer trips is factored in.

I've seen this time and again over the years, and often just for bog-standard cars just because they are shod on low profile tyres and have sports suspension for a 'Sport' model that really is a sheep dressed in wolf's clothing, or something 'fashionable' like a (BMW) Mini Cooper.

So many 1-2yo used examples of these types of vehicles which obviously aren't ex-fleet / hire / ex-demo cars appear back in showrooms these days.

This is precisely why an extended test drive, and preferably having such cars for a whole weekend/week (if possible) is a great idea, so prospective buyers can see what they are getting and whether they can live with it.

They can also take some time to get info on costs to see whether they can actually afford to run it, especially if the car /brand has reliability problems and they could be spending shed loads of money on just keeping an occasionaly (sunny) weekend car roadworthy.

This is even more true for convertables and especially older examples. Some will likely leak or rust whatever you do or at the very least need a lot of regular TLC, either taking a lot of your own time and effort to DIY, or a lot of money if leaving to a third party, neither of which will guarantee success.

A former work colleague buought a DB5 (admitedly this is essentially a 'worst case') and ended up spending a six-figure sum putting it back to its former glory.

It is worth about 5x what he spent, but if you budget wrong, you end up running out of cash without the car being finished and likely not worth what you've paid out even if you sold it.

I can't think of a good reason. Might be worth checking the MOT history on any specific cars - you might find, for instance, that someone ditched it as it had advisories, or they thought it would fail, or that they didn't want to risk it. A subsequent MOT would then show those issues have gone away, which suggests someone has decided to sort it. You can find an interesting narrative sometimes...

Average ownership seems to be about 2 years these days. People seem to get bored very quickly.

I wouldn't worry if the average is in the 2-3 years per owner bracket or even slightly higher if passed through a dealer who registered it etc.

Certain types of cars - sports cars, convertibles etc also tend towards high ownership as people buy them and discover within 2 weeks they don't work in their lives.

8 owners in 10 years or less for a regular car is on the high side however and would make me think twice.

Rather like potential spouses then? :-)

Back in early 2009, we bought a 4 month old car and became its third owner. First was Ford (it was a Ford Direct car), second was the dealership we bought it from. The dealer principal had planned to use it as his own car for a few months so registered it in the dealership’s name rather than running on trade plates. But we saw it and he sold it to us. My wife keeps her cars usually for longer than me, and we kept that one for 6 years. I typically change my car every two years, so a car like that, with me as the owner, and it would be a high number of owners in a short space of time.

To be fair though, you probably (as a lot of people do) buy new or nearly new and then change at 2 years old / later. Most people buying a 6yo car won't change it at 8 then buy another 6yo car, they're more likely to keep it for a lot longer, often until it either expires or has a very expensive repair on the horizon.

I, for example, bought a 2yo Micra as my first car, and kept it until it was nearly 10 years old, px'ing it for my current (essentially new) Mazda3.

I suspect that the average 10yo car would've had 2-3 owners, discounting any 'transfer' as you spoke of, which was similar to my Mazda, where the original 'owner' was a lease hire firm (no-one hired/used the car, hence why it was technically still new) who then sold it on to Motorpoint, who flogged it cheap to me. Technically I'm the third owner, but in reality I'm the first.