Often they are what they call in the timber industry, widow-makers. A limb, and can a realy large limb, has broken in the past, fallen part way down an then became hung-up just waiting as the stresses of gravity continue to pull and supporting branches begin to yeild util they fall, often only producing a rustling of the branches.
I had a good friend killed in HS. Big yellow Lab in the neighbor hood ran out infront of him.
I always like to say to those who say the are "safe riders" no such thing, only cautious riders. If you ride you are not safe. I have been hit five times, once by a fellow biker. Three times while at a stop sign. Riding in traffic is like playing foot ball and once ya get the ball all your team mates switch over to the other team. They are all out to get ya. Of course I have been riding over 30 years and have logged close to 400k mile. Two wheels has always been the way to travel, rain or shine, even snow.