I am not 787 qualified so cannot comment on the specifics of this. From a general perspective there may have been several things that could have happened, hot brakes can happen on taxi out, dependent on many things, it is not unusual for brake fans to be used on taxi out on SH aircraft to keep brake temps down. It can be interesting to keep a light aircraft speed under control without heating the brakes up considerably, - a worn or unserviceable brake unit or brake temp gauge can make this happen. On some aircraft u/s brake temp sensors or indicating systems can lead to when being left down after take off to ensure that the wheels are not too hot on retraction. I have a vague recollection of an RAF VC10 having a wheel explode in the cruise causing massive damage to the wing with subsequent hydraulic issues, hence the caution with hot brakes on take off. It was probably something of that nature but as I say I am not 787 qualified so I can only speak generally.

Just found something on that VC10,

"Over Birmingham a large bang was heard from the wheel well. The damaged tire had exploded and the debris from this took out the fuel lines to two of the engines and the hydraulics on the lefthand side. Also, because of the fuel valve selections that are made to balance the fuel load in such a situation, all of the 15.000 pounds of fuel in the port wing disappeared down the damaged valve cluster in the main wheel well, leaving the aircraft low on fuel during the approach. It landed without further incident at East Midlands though."