More than one person has noted that transactions have a much higher probability of being classified as "high risk" if the exchange rate has gone up significantly between the time it was started and the time they credit your account with bitcoins. One would think they allocate the bitcoins immediately at the time they lock in the price, but if they aren't doing that it would make sense for them to cancel the order if the price has moved too far.
That would certainly change the classification of this from being unprofessional to being unethical, so I didn't want to say it myself, but the thought did cross my mind. (There has been an 18% increase in the exchange rate since my order was put through.) What's also rather irritating is they still deducted the USD from my bank account. So they didn't really "cancel" the order. They just canceled the part of the transaction where they provide the product they've been paid for. I'll be curious to see what happens with the rest of my pending orders, especially the ones placed at higher exchange rates.
I really do want Coinbase to succeed. A functional service like this is very much needed, and their setup may even be easy enough for Grandma to use. But, the real world has very little tolerance for the kind of shenanigans that seem all too common with Bitcoin "businesses".