Can’t see why you are quadruple puzzled.????(re partition of Spain if Catalonia secedes)
Sometimes this is enforced by external agencies as in India/Pakistan or the two Koreas. Sometimes it is by mutual consent between equals as in Czech Republic/Slovakia. And sometimes it is by secession of a smaller unit from the larger union as in 26 counties and possibly at some future point in Spain or even Great Britain.“Wikipedia” said:Partition is the division of a territory into two or more autonomous regions
It is not necessarily a bad thing but where it has negative repercussions it is natural to look for who was to blame for the partition. In the case of secession of the smaller part that blame must mainly accrue to the secessionists.