What caused Tangmere to slip so badly?

I know that Bulleid pacifics have a reputation for being light on their feet, but what specifically caused Tangmere to slip to a standstill on the gradient south of Deal on the 11th November? She was all but 4 hours late by the time she got back to Bromley. I filmed her going great guns between Marden and Staplehurst that morning. I would have thought that a load of 10 bogies would not have caused her that much trouble.