I sat on a trading desk for years and this after hours indication was nothing but a trader settling up his books for the day and year. This trade could have occured at any time and is meaningless but the trader/sales-trader who wrote the ticket had to put it in after the close and he took down the other side himself in all likelihood.
After hours trades were real. I picked up a whole 300 shares @ $8.03. then the trader hit everybody down to $7.60 real slow. Over about a 15 minute time frame. Seemed like a margin call to me or perhaps some end of year profit taking. Though the latter seems insane since it was about 5000 shares down there. Anyway...somebody picked up a nice easy scalp for Wednesday morning at the open.
nothing to worry about, obviously if that is true(haven't looked at AH market today) somebody just needed to get out this year, and as several others posted before, it doesn't matter when it settles it is when you trade. 20k is nothing when we have over a million daily average, now a year ago that may have meant something but now...white noise, imho.