Why do blueberry smoothies congeal?

My blueberry drinks (after only a short while in the fridge) congeal. The solid floats to the bottom and yellow liquid floats to the top. What - specifically - in the blueberries causes the drink to congeal, whereas other berries don't have the same problem? Canned blueberries don't do this... Surely there must be a way to prevent this from happening while still using fresh blueberries?