I'm led to believe that the 0950 X93 from Middlesbrough broke down on a country road en-route to Whitby, meaning that the 1020 was forced to carry two double decker's worth of passengers.
Unsure whether it was a B7 or B9, but I'd guess the former given that it was one of the summer extras.
RE: Arriva North East: Latest News & Discussion - May 2015