can we explain a single actor buying 5k coins?
As mentioned, margin call, or, maybe even more likely, plain old fashioned belief in future price increase. I'm firmly in the bear segment for now, but even I can see how it might make sense to buy in now with a large sum (at the risk of short-term losing some of your depot's value) under the assumption that the long term growth potential that outweighs that risk is still there. It's all about different time frames in which market participants calculate (and, no, I don't necessarily say those who think "long term" are making the smarter choice).
On bitfinex there really hasn't been a large amount of short sellers compared to the number of longs that are being made. Usually you would expect a trend reversal when shorts out number the amount of longs, causing a short squeeze, which would somewhat indicate a trend reversal. There are far to many people right now betting that the price will bounce up immediately paying .2% interest a day on bitfinex.
Don't know how fine grained the data from
http://charts-bfxdata.rhcloud.com/bitfinexLiquidityBTC.php
is, but in principle we should be able to see it in there in a few hours if it was short squeeze. But I agree, not the most likely explanation. I go with attempt at pump&dump (not all that likely either), whale trying to stabilize price (seems like a lost cause to me), or simply: buy in by large actor (most likely, and mildly bullish)
