You get stuck back at the point where you became confused and cannot progress beyond that point. Of course, some emotional trauma is much the same way. Until one can understand, on whatever level, the significance of an event, there follows digestive problems and perhaps the accumulation of phlegm in the body. Left long enough to fester, and it can become a cancer. In the case of blood stagnation, the cause might more relate to one's sense of self.
Blood is among the deepest, most personal of all materials in Chinese medicine.
It relates to one's sense of who they are. If who you are is inconsistent with cultural, or even personal expectations, it will stop moving well, and perhaps begin to collect in one particular area. The area that it chooses will likely be consistent with that area of one's being that they feel is wrong, at least on a metaphorical level. In the case of a brain tumor, because the brain is an extension of the Kidneys in Chinese medicine, the blood stagnation might be due to one's disgust with their own race, something that deals with ancestors and descendants. Kidneys hold the Chinese equivalent of genetic material.