There is a big difference with forgetting a small object on a vehicle roof (like this specimen hypothetical) and a giant boulder. Manslaughter does not require intent, but it requires that the initial conduct was willful (done on purpose) and a reasonable person should know that there is a huge risk of injury or death because of that conduct.
In the case of the boulder, anyone can tell that a large, unsecured object could easily cause serious injury or death if not secured properly (since it required equipment to move in the 1st place). In the case of the small portable rock specimen, the conduct was an accident(forgotten object) and the chances of it coming off the vehicle in such a way to lead to a death is >0 , but only fractionally so.