Yeah, that looks really weird; when lowering, the bag levitates as if the string is a stick.
The animation also ends too late. By the time the trebuchet launches its boulder, it'd be firing at the ground.
How about some dust particles in the death animation?
In the Morph Alternate animation, rather than spontaneously levitating and floating in a wide arc around the entire trebuchet and also clipping through parts of it, how about having the disc stay attached to the end until the rest of the trebuchet has been packed up, and then have it rotate over the top and turn over flat (after moving, not during; that would look weird).
Aside from those minor flaws, 4/5; I'm sure you'll fix the issue with the string, that's the more important one.