More details have emerged from the launch of Delft Aerospace Engineerings Stratos III rocket. Those who watched the stream know that at about 20 s into the flight the rocket encountered an anomaly.
More specifically from the teams latest blog post,
The rocket trajectory was a straight line from the 85 degrees launch rail and fit our simulations very well. The anomaly occurred at around 20 seconds into flight. Stratos III was flying at around 11 km with a speed of nearly 900 m/s or close to mach 3 at the time of the anomaly. In addition the failure occurred so fast that data transmission was cut of almost immediately. Some sideway 2g acceleration was registered by the on-board inertial measurement units however this still needs to be confirmed as the rocket was also spinning slightly.
Whatever happened it happened at Mach 3, now that is fair hauling!
The team is still investigating, it will be interesting to see the conclusion and how this will be implemented in the next generation rocket.