A lot of rubbish being spouted here
Rumours are one of the sheer pins failed, tug driver attempted to slow down by using the brakes which resulted in the remaining sheer pins going, and tow bar head becoming detatched - Subsequently allowing the aircraft to run into the tug.
The Boeing 787 could be a problem for this procedure as I believe it cannot start the engines without an external power source such as FEGP or GPU. Or maybe that was just peculiar to the 3rd prototype which we handled on a sales trip?
787 can start it's engines with the APU like any other aircraft can