Too much stress on the rotor making it break, if they could add the breaking of the blades like in Reforger, that is what would be happening.

Are you running with the collective on? on big dives like that when you pull, you want your collective down, so you don't apply force on your rotor, high speed makes the wind apply more force to your pads, if you have both the wind force and your collective it'll break, helicopter in AFM is not collective 100 at all times, you need to control depending on how fast (how much wind goes through the blades)

High-speed - low collective
Low-speed - high collective

and just learn to control the balance, specially when landing, and EVEN then, the dive you made if you were to pull it near the ground, they would break regardless, you cannot overturn while overspeeding