Well its a lot of things, you could bend the wire beyond its specs for bending radius and in a way that would damage its ability to carry current. You could install it in a way that puts the cables too close together so they then transfer heat to each other and overload the thermal rating of the insulation, thus causing a failure there. You could damage the insulation on the cables leading to water ingress. Proper to code installations which provide plenty of margin to prevent fires are defined mostly by the NFPA 70 (National Fire Protection Act) National Electric Code

You could install the cable in a way the the heating and cooling of the cable, over time, causes it to move or shift (kind of like a snake, generally in the direction of gravity where its on a slant or hanging down) and then cause tension on connectors that wasn't there before, if you didn't properly secure the cables or design with that in mind. This one was an actual issue I know about having happened on some wind turbines.

If you have a really incompetent contractor, maybe they used the aluminum conductors instead of the copper conductors in some places (the former is far cheaper, but has less bending radius and carries less current).

There is a lot to get wrong, most of which I had no idea about until I had to think about it and know the problems in that domain of engineering. Even then, I only did that for a short time so I'm sure there is plenty I don't know.

Most of the issues I heard about though weren't engineering issues, it was poorly trained contractors who had no idea what they were doing. They used contractors especially in the Wind Industry, because of the cyclical nature of the building of power plants. Wind had a tax credit that use to not be a partisan issue, then it became one and congress couldn't get its act together which caused the sudden stop of ordering of wind turbines several times. Then there is the lack of unions in many parts of the country, and unions generally keep their workers sharp by keeping them employed even in down turns by evenly spreading out mandatory furloughs. They also go through rigorous apprenticeships and trainings.

Not necessarily. Micro-converters to step up from panel DC to a common DC bus, and then inverting from the common DC bus to house's AC bus is much cheaper than going straight from DC to AC at the panel.

My house is wired up this way, with individual DC/DC maximum power-point trackers at the panel, and a central inverter.