what causes a transmission leak and is this a common problem with car?

In my case, the transmission hose for my 1998 Chrysler Concorde ruptured. The hose is a combination of aluminum and rubber. The rubber failed. No hose lasts forever, so it's good to have all belts and hoses inspected regularly. Even that's not foolproof, but if you can catch it before it breaks, gets a hole, or splits, you can save yourself some grief. I was not so lucky.