It's not clear whether this was truly a hack (Coincheck) or the exchange was already insolvent due to a prior hack and the operator Francesco "the Bomber" Firano tried to cover up the whole thing with his tactics to blame the XRB dev team followed by forcing identity verification and stopping fund withdrawals.

Some interesting posts show there were several coding bugs in the exchange software which allowed for price manipulation by crashing the price through price manipulation. e.g. Apparently you could put in a 100K buy order at a low price, put in a sell order, and this trade would go through. This would tank the price and other people panic sell. There were probably many more glitches and exploits in their poorly-designed exchange.

Sources:
https://np.reddit.com/r/nanotrade/comments/7wklxm/bitgrail_hack_very_likely_came_from_btc_ltc_eth/

https://www.reddit.com/r/RaiTrade/comments/7n0ou8/an_explanation_of_how_the_shitshow_that_youve/