So, I was there, casually playing as Ragnarr Lodbrok of Scandinavia, comfortable in the thought that I would soon come to pass to Valhalla and leave the Empire to be divided by my four sons according to the laws of Elective Monarchy (where my eldest was the primary successor). And then things happened.
First of all, my first son, Sigurdr, inherited the Empire of Scandinavia and Kingdom of Sweden. My second son, Snorri, inherited the Kingdom of Danmark and the freshly succession-created Kingdom of Norway. He remained a loyal vassal of the Empire of Scandinavia. However, my third son, Gandalfr, gained independence with the duchies of Austergautland and Finland. As did my fourth son, Thorolf, with the duchies of Estonia and Vestergautland.
So, I did what any good loving brother would and I quickly proceeded to re-annex the domain of Thorolf to unify the land. And then things went even more wrong. First of all, Gandalfr declared a Subjugation War on the Kingdom of Finland (consisting of Karelia, Kola and Livonia) which he also surprisingly won. This resulted in an independent Kingdom of Finland (and Austergautland) under the rule of the third son. I had already reclaimed Estonia and Vestergautland by this time.
However, due to some anger-management issues the second son had, Snorri accepted a duel challenge from one of his vassals - and lost, forgiving his life. This resulted in the Kingdoms of Norway and Danmark being passed through four holders within a single year, eventually ending in the hands of the now-independent king Gandalfr of Finland, Norway and Danmark. Currently, the Empire of Scandinavia consists of Estonia and Sweden, while the third son rules the rest. I've already managed to place him as the heir to, in case of accidents.
For one, should this even happen? For two, how would I go about preventing this from happening again? And lastly, what is the best way of speeding up the reunification process on my end?