Intussusception occurs when a section of intestine over-rides another section, trapping it inside in a telescoping effect. This cuts off the circulation to the sucked up section of intestine and it dies if the situation is not relieved. The dead section of intestine then leads to the death of the affected dog as infection and shock occur.
Intussusceptions generally occur as a secondary problem to some disorder or disease that causes increased intestinal mobility or inflammation. Chronic parasite infestation may be the most common cause but anything that causes chronic intestinal disease can lead to an intussusception. There are times when intussuceptions occur for no apparent reason so it is likely that primary intussusception can occur. They have been reported following ingestion of foreign bodies, trauma, ovariohysterectomy surgery, infiltrative or inflammatory bowel diseases and other conditions that cause intestinal irritability.
Intussusception can be chronic sometimes. It can also be a "come and go" problem -- intussuscpetions occasionally will resolve on their own and then recur. I have removed one surgically, put the remaining intestinal ends back together and watched a new intussusception form right at the same site.