This was not a technical issue. Everything worked exactly as designed. A large market sell order reduced the price to a level that triggered other sell orders (a combination of "stop-loss" and forced liquidation for margin positions) in a cascade. This was a flash crash that was over within a few seconds, so other markets did not react.

The issue is that there are a lot of inexperienced traders on GDAX right now and they were recklessly trading on margin and/or setting stop-loss orders without understanding the possible consequences.