No explanation, but I contacted Uber Support via Twitter, and eventually received the following response, after a few scripted replies:
We understand that this has been frustrating. We've escalated this to the appropriate team, who will follow up with you as soon as possible. When the time comes, please respond to them through the in-app help section or by e-mail. Thanks for your patience while we look into this.
They determined that "as a security precaution, a temporary hold was placed on your account". They made an adjustment to my account which resolved the issue. Hope this helps!