From experience, Coinbase automatically flags your account when anything fishy happens. I have had this happen three times now. First Chase closed my account without notice due to Bitcoin dealings. Then Wells Fargo. Then PNC. Each time Coinbase attempted to execute a trade I had placed and the bank account was frozen, my Coinbase account was automatically frozen as well. I then had to contact customer support via e-mail and explain the situation to get my account reinstated. There are several things that could cause your account to get flagged though. This is all automatic and has nothing to do with the price.
What likely happened after the first time is that coinbase somehow flagged the transaction as high risk to your bank who then went and closed your account to prevent risk themselves. This would cause the coinbase transaction to not go through, making you look even more risky in their eyes.