Its really hard to see your pic, but did the tube seperate from the casting? If so it could be from abuse, or from sitting outside for a long period of time. Ive seen some where the tube is full of water, and the po never greased it......ever...and they rust from the inside out. If you can get a better pic we might be able to help a bit more, its really hard to tell whats going on there.
Jeff (teet)