What’s up with those rock climbers who look honed as hell, but the minute they step off the rock, they turn into the Hunchback of Notre Dame?
Grectmeme | Rock and Ice Forum
Climbers, especially those bloody Young Ones, are generally lazy bums unable to give energy to anything that is not climbing. This includes healthy posture. They slink around, keeping a low profile on the fringe of society. Eventually that posture becomes their identity. The body follows the mind. I could be wrong (but probably not!).
The other possibility involves the interplay of multifarious biomechanical forces that would take a thesis to explain. Basically it comes down to a chicken or egg situation between pelvic tilt and thoracic curve. In climbers, tight internal rotators at the shoulder, the result of always pulling down, are a likely catalyst by pulling the shoulders down and forward, but even this is far from a given in any particular case.
Likewise, the solution is wide and varied. The simplest approach is to do yoga (kama sutra!), cross train (defense techniques like Brazilian jujitsu and Oompa Loompa mud wrestling are especially effective), and whenever you remember: Chin in, tits out!