By Sinead Cruise LONDON (Reuters) -The sudden fall in European shares driven by a brief crash in Nordic markets was caused by a single sell order trade by Citigroup Inc, people familiar with the matter told Reuters, and confirmed by Citi. The flash-crash, which caused European shares to suddenly fall on a day of holiday-thinned trading activity, involved an erroneous calculation relating to a Nasdaq Inc index involving Swedish companies, one source said. The Stockholm OMX 30 equity benchmark index was one of the hardest hit, falling by 8% at one point following a five-minute dive, before parin…