I have experienced a similar thing with my discus when the largest started to be quite aggressive towards the other 3. Not sure what the shaking thing is ( although i have noticed this too) . since moving them out of their 180L tank and into a 500L tank there is no aggression at all and have even started loosely schooling when moving around the tank.
My guess is that the aggression comes from the limited space in the tank along with how discus will naturally form a pecking order that can cause some aggression towards the smaller fish if the largest discus feels it wants more space. Try adding more vallis or other hiding spots to help give the other discus a break from any aggression.
I would be interested in finding out what the shaking was too though.
- chocky likes this