Cars can total for different reasons. Could have been in a wreck- most insurance companies total at around 80 percent. Say the book on it is 5 grand, it only takes 4 grand to total it. So basically if it was rear ended and needed a quarter and rear body then it would probably total. With the price of parts it doesn't take much to total cars under 10 grand. Floods also total cars. If a car comes into our shop that was in a flood and now won't start, a lot of times insurance will total it simply to avoid the hassle. Cars can also be structural totals, such as when a firewall or cowl panel is crushed. You've also got situations (like we just had at our shop) in which a Mercedes is made of aluminum. The problem is you have to be certified by Mercedes to work on aluminum. As far asi know there's only 1 shop around and they're constantly backed up (due to being the only certified shop). So rather than let a car sit and accrue rental fees, the insurance company will total the car.
In short there's lots of reasons a car can total. None are good but some are certainly worse than others. Always get a Carfax or similar report when buying a car. I'd you're serious about getting the car then it's well worth spending the 50 bucks for piece of mind. Now of course not everything is reported to Carfax and some cars that would be a total get fixed for less money to keep from being totaled, but at least you're not gonna have something like this happen.