[wstein]

Emotions that are not released get stored as tension in the body. Most of this is muscle tension. When you do yoga, especially postures, those muscle tensions are stretched out and physically released.

The mind is associative. When events lead to stored emotions, the mind remembers the association even if the conscious is unaware of it. When those tensions are released, that association can bring back the memories of the original event(s) that caused the tension. Particularly intense releases can cause one to relive the original event(s).

Note that if the intense emotions are not dealt with, a future event similar to the original (even if to a third party) can bring back those emotions (via association). As such, the tension returns also.