To be honest whole thing took only 3 minutes before trading was frozen, so I could buy the story GDAX did not know this could have happened. If this was done on purpose trading volume would be much much bigger. But who knows...
You have to take a look at the Bitcoin Cash exchange rate development at the other exchanges before
the Coinbase launch.
Basically, BCH had a steady ~30 % surge before the Coinbase launch. A chart pattern like
this literally never happens unless someone is very sure that good news are about to be released.
This is what is fuelling the insider trading allegations and not the short trading action
at Coinbase until they halted trading. The run of Bitcoin Cash at other exchanges
in the hours before the start of BCH trading was the suspicious event.