Nobles getting too full of themselves. Their nobility is GRANTED to them by the Crown, so attacking the Royal Family just because a Prince doesn't want to give up a relatively unproductive, tiny town of backwards, ailing, dying people should seem like absolute retard-strength madness to them.
They are literally sending an army to 'teach him a lesson'. Whoever gets the throne, even if it happens to not be him in their perception, would have their titles and lands stripped, heads of family hanged, and the their offspring sent to work the mines. If some mere nobles manage to kill a prince and the Royal Family does nothing it means they're ceding to the nobles and thus all the nobles should make a bid to take the Crown for themselves leading to chaos and civil war, squabbling over a throne that still has a butt in it.
Point is, whoever wins the throne isn't going to allow nobles to get away with attacking royalty. Otherwise they lose all credibility and the position of the royal family becomes equivalent to crap.