There are many factors that can cause an airplane to snap. Basically one wing is stalling before the other. CG does have to do with how violent and what airspeed the snap, but it is not causing the snap. A forward CG will cause and airplane to stall at a higher airspeed. An aft CG likewise will cause a stall at lower airspeed. Your problem is that one wing is reaching the critical angle of attack (stall) before the other. It may be cause by incorrect incidence or maybe a twist in the wing. If you have an incidence meter check bother wings and make sure they are the same. Also check your elevator(s) to make sure that they have the exact same travel in up, down and neutral positions. You want both wings to stall at the same time. Having a forward CG will just make the stall occur sooner when you apply backpressure.

Hope that helps you a bit.