Here is the link to the Coinbase Twitter feed where they make 2 announcements that they were having problems, oh and notice the time of the announcements, just as ETH was hitting $253 and there wasn't a single announcement about any issues before that. The most recent announcement before that one was on June 12th which was Monday, 2 days earlier.
Announcement 1 6:30am https://twitter.com/CoinbaseSupport/status/875345900779954177
Announcement 2 8:30am https://twitter.com/CoinbaseSupport/status/875374176894201856
And here is a link to Coinbase twitter page so you can see the timeline of all of their announcements
Also here is a link to cryptowatch Coinbase ETH/USD so you can see the timeline of ETH/USD and compare it to the times Coinbase announced they were having problems. https://cryptowat.ch/coinbase/ethusd/30m
Whether or not it was an actual attack or done purposely is totally irrelevant. The point is that you can clearly see that ETH started dropping at approximately 7-8PM Wednesday June 14th and continued to drop for around 12 hours before abruptly coming to a halt at 7-8AM Thursday June 15th, ironically or coincidentally at the same time Coinbase website shut down and they made announcements on Twitter. So ETH was dropping for an entire 12 hours before coinbase made any announcements. The timeline of events is clear as day.