Usually Coinbase goes down when the price goes down, not up. Probably because a ton of people are trying to sell their bitcoins as soon as the price starts walking downwards.
The big difference is now they have lots of competition in US.
Yeah it seems to be more an issue of volume than the price direction.
An old joke among Bitcoin speculators was "All technical analysis in a nutshell is basically: Bitcoin may go down, but Bitcoin may also go up. Time to add a new punchline: On the other hand, we're all sure that Coinbase always goes down.
Bitcoin is one of the rare instruments with inverted skew in its options i.e. volatility is higher when the price is going up. So we would expect outages to be more correlated with price increases than decreases.