Manipulation would mean a deceptive action, but moving walls around happens in public. Besides the psychological effects, there might even be an element of rationality behind doing this: you can suddenly change the total margin value of other margin traders, and potentially cause forced liquidation on some of them. That is, playing by the rules that everyone knows, moving walls around could sometimes be a reasonable strategy even neglecting psychology.

Sure, playing games like this causes short term price swings that do not reflect the fundamental value. But hey, the value of a commodity on a market always is a mix of its fundamental value and some "playing games" white noise - games that often are not manipulative (deceptive), but playing by the rules in some creative way.