That thread contains numerous people claiming their USD withdrawals were marked as completed, but the money failed to appear. Overloaded customer service reps from Coinbase appear to be looking into and handling them individually.

I think this is what it would look like if their systems had been hacked, and thieves were cleaning them out by changing the destinations of wire transfers. If that was happening, they wouldn't necessarily be insolvent - they were probably holding lots of Bitcoin during the massive rise in price, and the losses would be in USD - but it'd certainly mean they were in trouble.