Quite common actually and the leading cause of this is the fact that the human body is designed to flex more on the toeside. When on heelside, we don't have the range of motion in our lower body joints we do when toeside. Heelside chatter is always 90% due to being too stiff.
To help reduce this, use more dynamic movements; especially timing your flexion and extension. Use a down unweight for making your edge changes instead of an up unweight that almost every high level intermediate rider uses. The down unweight allows for earlier and more positive edge engagement early in the turn. As you work the top of the turn, slowly extend to set that edge. As you pass through the apex of the turn, you need begin to flex again. This flexing action makes your legs more loose and this will absorb a lot of that chatter. Anytime chatter happens, bend something! Always get loose to quiet chatter.
Typically, people hesitate on toeside turn initiation and this could be happening to you. The last statement you made also clues us in to where a problem may lay. In trees and bumps, your shoulders should not be aligned with the hips knees and the board. Tree and mogul riding requires the most dynamic riding skill of all. There should be a lot of upper and lower body separation to make these short radius dynamic skidded turns in this environment. Your shoulders should be pointing down the fall line while you allow the lower spine to swivel. The fully aligned turn is not effective enough to control your board in the conditions described. Get more dynamic and allow upper and lower body separation.
This is called using good fore-aft movements and these movements are critical for making smooth, controlled turns on steep terrain. It only takes a lack of fear to straight line a steep, not skill. True skill is being able to make fairly consistent turn shapes on gnarly steeps. Among other thing and the thing the CASI instructor as eluding to is this fore aft movement.
Basically, as we all know when you initiate a turn on steep terrain we need to get more flexed and forward toward the nose. Problem is, the intermediate ride believes this where they need to ride all of the time. Nope! There is a time to be aft on your board and that is steep terrain.
So, shift to the nose to initiate the turn. As the board travels through the turn, before apex, shift your weight aft. This will feel like you squirted the board out ahead of you and you are riding the tail. After apex through the bottom of a turn, you want to be aft to prevent the tail from skidding and it is much more effective to finish a turn while aft.
Hope that helps actually open up more questions for this is the tip of the iceberg when it comes to advanced dynamic riding....