Hmm. It is generally standard practice for masters of vassals to want to install their own successor as soon as possible, this is to stop the vassal from deteriorating from the lack of a government. Normally, the master would not assume that the vassal could be capable of appointing a temporary ruler (especially in these circumstances) since other nobles of the vassal may fight over the position, or, even worst, attempt to "prolong" their rule indefinitely (there is no evidence that someone from within the vassal was not responsible for the assassination).
So, to ensure that they (the masters) control the situation rather than squabbling greedy nobles, the master would send their own temporary ruler to create a new government and then withdraw when the heir to the throne is old enough to rule (or, if the master is so inclined, "push" out the heir from the vassal in a number of creative methods designed to put a positive spin on the heir being removed from the vassal, if the heir is to be hostile to the master).
*Giggles* I could go on like this, thinking up all kinds of plots, but I think I have said enough.
