These facts suggest crew hours. The aircraft made it in but presumably there were not enough hours to make it back. On long haul flights, this is generally known in advance. On short flights such as this, there are simply too many variables, so it makes sense to just keep pushing the flight until it is clear that there won't be enough hours.

In any event, as to your original question, it is often the case that there are multiple factors which stack up. So, trying to figure out what exactly happened is simply an exercise in futility. It only matters in terms of whether the reason is within AA's control and thus AA is on the hook for a hotel voucher.