Let's think about this. Would cracks in the body of the comet cause it to break apart and string pieces along behind? For the coma, the tail, the particles are very tiny and solar wind can move them easily, but the bigger pieces that can be seen individually would not be so easily deflected.
Another thing is that the pieces stay in a cluster, a peloton, rather than being further moved from the cluster, unlike the tail that spreads out forever.
So what forces the chunks apart to begin with and then stops acting? It's the volatiles that are thoroughly mixed in with the body of the comet. A piece of ice down inside the body of the comet would remain frozen until the chunk it is attached to breaks free. Then the piece of ice begins to evaporate and the gas acts like an ordinary rocket exhaust slowly pushing the chunk away from the main body for a while until the ice is evaporated completely.
Differential thermal expansion causes cracks, but ice and other voilatiles provide the motor.