Some more inconsistencies: the stick's length changes ie when it begins it has a bit sticking to the right of the pivot point, but at the end it doesn't have that same bit. Also, the width of the stick should not change. ie. in that frame basara posted, its one pixel high, but in the first frame it is two pixels high.
About the design of the trebuchet, having that counterweight (a wooden box filled with rocks) suspended by a rope seems unlikely. I'm no engineer, but I a doco I saw on trebuchets they had the counterweight attached to the arm using wood. I wonder if its on youtube (I saw it on tv). The rocking forward that you have shown was reduced by having the trebuchet on wheels. That doco would be great to see the timing of the rocking. I believe your timing would be wrong. The mass of the rock they are flinging will be very small compared with the mass of the counterweight, so I don't think its release would be the cause. Instead of rocking it forward when releasing the rock it should rock forward as the counterwieght goes back (centre of mass should be conserved). Last frame projectile's velocity is inconsistent.