I do not have knowledge about the particular accidents stated in the question, so my answer is directed at rope soloing in general.
The fact that more reported accidents happen when rope soloing is moste due to the fact that it is soloing. While in a team many minor accidents can be handled by yourself, so there is a huge number of accidents that do not need rescue and therefore never get reported. When soloing you are almost certainly stuck when anything happens, so you will need assistance. Therefore the number of unreported accidents is much smaller.
The second factor is the higher complexity in the system. You have more and complicated gear to handle (just read the question linked, there may even be stuff like modified grigris involved). The amount of errors made always scales with the complexity of the system. Even in the less complex system of climbing in a team, it is highly recommended to do partner checks at all time to introduce redundancy and thus find errors. Because knowledge and experience does not prevent errors (some studies even state the contrary effect, i.e. habituation increases the chance of errors), the fact (true?) that only well trained people do rope soloing does not help either. Again in rope soloing you do not have a partner with a second set of hands, eyes and another brain to check what you are doing. And I am not ashamed to admit that this second brain potentially saved my ass several times in the mountains.