First, a definition: Black Friday is the Friday after American Thanksgiving (fourth Thursday of November), which is the popularly acknowledged start of the American Christmas-shopping season. Many people in the US have the day off work (or school) and retailers put on big sales, so the (AmE) malls/(BrE) shopping centres are heaving with people and unseemly behavio(u)r.
Why is this the US-to-UK Word of the Year? Because it was all over the UK media in the week of (and the week after) American Thanksgiving. (Three readers contacted me to nominate it--the only nomination I had this year.) Here, for instance, are stories from The Telegraph, BBC News, The Mirror... (I would go on, but I'm sure you can google "Black Friday UK" yourself.) As well as 70% discounts, there was bad behavio(u)r (which you can find easily enough by googling "Black Friday UK bad behaviour").
It is an annoying import because there is no logic to the importance of this day in the UK; it is a regular work/school day and the Christmas shopping season is already well underway in the UK by that time. For instance, the Christmas lights were ceremoniously turned on in Birmingham city centre (AmE downtown) on 9 November, on 14 November in Brighton and London's Oxford Street, and on 21 November in Guildford. (I could go on, but you can google "Christmas light switch-on UK" too.)
What has brought "Black Friday" to the UK are US-owned retailers, notably Amazon and Walmart-owned Asda (site of much bad behavio[u]r--click the link for a story about that). These retailers are partial to some bad behavio[u]r themselves, such as union-busting (quelled at Asda, I should say), tax-evasion, and (particularly at Amazon) poor working conditions. So, American phrase, but please don't blame the average American. If you don't like it, then I recommend doing all your Christmas shopping for the following year in the very British Boxing Day sales and avoiding this whole sordid Black Friday business.