It’s also about the burden of proof, it’s easier to keep someone jailed on manslaughter.

Manslaughter is definitely a lesser charge. Usually used if the death was accidental or such.

Just to clarify on this: There are (in english common law typically used in the united states) 5 mental states associate with criminal action: intentional, knowingly, recklessly, with criminal negligence, and accidental (sometimes you'll hear it casually as innocently). There is a great guide on this called 'The illustrated Guide to the Law' that everyone should read. It's a website, just give it a search.

Intentional and knowingly are almost always considered one and the same, because proving that someone committed a crime with the express intent to commit the crime is actually really hard if you are following logic, but proving that someone knew an outcome would be the outcome for the action they took and simply did it anyway is a much easier task. Intentionally and knowingly are the mental states associated with what most of us would call murder.

Recklessly is associated with what texas is calling manslaughter. Recklessly boils down to you knew that there was a risk of the outcome occurring, and disregarded that risk and did it anyway. In this case, the state will have a MUCH easier time of proving that the driver knew there was a risk involving suddenly stopping on a road and backing up when there were other people/cars/bicyclists around, and simply carried out the risky action anyway.

If you're curious, the last state of criminal negligence is if a reasonable person would know that the action carried a risk, even if the individual doesn't recognize or know the risk.

Oh, and of course accidental means just that. Even a reasonable person wouldn't expect the outcome that occurred, and the person didn't know it. Think about the scenario of backing up from your driveway after checking behind you before even getting into the car to make sure no one is on your driveway, but one of those wild runaway prams got behind your car as you started to back up, and you knock it over and the baby falls out, hits its head, and dies.