Capital moved on character death?

(ck2 per the flair) was playing as 1066 count of Aruoane/Targhaza (basically under Maghreb/Amoravid but initial capital was Arouane in de jure ghana. I got a duchy in Maghreb (which Targhaza was part of) and also through fabricating claims conquered murcia (in spain). i moved capital to murcia. Later when i died my first son inherited and the capital moved back to Arouane i think (which isnt even de jure part of my duchy... why did it move? Having trouble finding the mechanics for this in the ck2 wiki. If anyone can point me to how this works that would be awesome.