[EDIT] I just watched the video... Right around the 1:30 mark, you can see the forward section try and separate. If they had a short shoulder it would have been quite a problem. It also could have been caused by a lack of venting... As the altitude increases, the air inside expands if it's not vented, and that can also push the rocket apart.
If your rocket is designed to break in the middle for parachute deployment, you need a tube coupler that is firmly attached to one portion of the body tube, and can slip inside the other body tube. If your rocket is designed to pop the nosecone, again, you need a shoulder that can mate the nosecone to the body tube. When a rocket is launched, everything is compressed together, and that's fine, as long as you don't exceed the strength of the materials. However, once the rocket begins to coast, Things with mass like to move forward. Here's an analogy... Imagine you're a passenger in a car, the driver really hits the gas hard, you're compressed into the seat. Once they let off the gas, your body wants to move away from the seat, and the seatbelt is there to catch you. If there's too little tension in the belt, you hit the windscreen. Now, back to the rocket. When the rocket decelerates, the mass objects (your altimeter, your GPS, etc) want to move forward. However, aerodynamic drag, causes the aft section to slow down faster... Too little of a shoulder, and the aft section separates from the forward components, the aft section drastically decelerates due to the increased drag of the now non-aerodynamic front edge (maybe even the parachute deploys), the forward section is still aerodynamic and traveling fast. If the shock cord isn't long enough, or the mounts aren't strong enough, the rocket will tear itself apart (or the parachute may tear open), resulting is a less than desireable return.
Also, if the overlap (shoulder) is short, and there's any kind of play between the forward section and the aft section, the rocket may buckle, causing it to tear itself apart. A longer shoulder helps reduce the likelihood of this happening.
The AV bay I linked to above is sealed from above and below, preventing the electronics from being damaged, and giving you firm mounts to attach your recovery devices.
I moved the LLs (and converted them to my interpretation of "rail buttons") to centered between fin, as rails may have a problem fitting between fins and rail buttons.