Stagnation can trigger another TCM imbalance. Energy that stays in one place for too long and can't move freely produces Heat, and if this goes on too long, this Heat can change into destructive Fire, and the person has some major health problems. Think of the analogy of an electric wire carrying more electricity than its diameter can safely carry. If it gets too hot this can cause a fire. Another analogy is a pressure cooker.
All pressure cookers have a vent for realeasing excess pressure.
If it didn't and the pressure rose too much, the entire pressure cooker would cxplode. Something similar to this can happen in the body when Qi Stagnates. The Liver and the Gall Bladder are the two Organ systems most vulnerable to anger and frustration. Something else which can cause Qi to Stagnate is Cold. Cold slows things down and congels.