The cause of chattering can also be several things such as: tuning, poor technique, insufficient ski for the skiers ability/intentions to mention a few in addition to what Ghost has already mentioned.
So depending on what is being described around the chattering you can start determining the potential cause, what can muddle the issue it can be one or all at the same time, so determining the solution can be challenging.
Example: My nephews beginner ski chattered and most thought it was technique until you saw when the chattering happened (had to chase him done the hills a few times to spot it), proper turn initiation, forward loading ski and they started chattering. The fault was the ski was trying to compensate for beginner (and not a intermediate or advance) skiers over input by releasing the grip (unfortunately it was intentional). Solution, higher end ski (Jr Race actually because of a deal), problem solved. The only chatter that happens now is when he doesn't get forward enough to set and hold the edge (again higher performance ski does not give you the lea way to give sloppy input). This chatter in particular looks very similar, but a better skier (instructor) should be able to identify the difference to help determine the cause.
Hopefully this helps clear some of it up for you.