This is ice breaking away from the vehicle due to vibration, acceleration and aerodynamic forces.
The Electron uses cryogenic liquid oxygen and kerosene as its oxidiser and propellant. The low temperature (approx −185 °C) in the tanks causes water vapour from the atmosphere to condense and freeze onto the body of the rocket. This can can be seen on the upper half of the first stage (where the oxygen tank is located) as a white covering which fractures and falls away on launch.
See this question for more details and examples.
Compare images of Electron before and after fuel loading where the frost build-up can be clearly seen and some falling ice is visible:
Left: It's Business Time, Right: Still Testing. Image credit: Rocket Lab