From the live stream, it appears that the 2nd stage motor started, then stopped a couple of seconds later. If so, I'm guessing a sudden loss of power to the electric propellant pumps.
ETA: Appears to have spun hard shortly after staging.
For small rocket motors electric pumps have a weight and simplicity advantage. However, a significant disadvantage is the reliance on batteries. Rocket Lab is the first to do this as far as I know. So time will tell if it's a net positive (no pun intended). Meanwhile, the exact cause of this malfunction is yet to be revealed.
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