POSCO/KEPCO or anyone else would not "openly" be informed what was taking place between SK and COK. They'd have their chance to respond after the announcement is made public.
I guess that's the other thing about the deal, it needs shareholder approval. The deal might actually get blocked, but I don't think anyone will block it unless they can provide an alternative means to funding.
The tree shaking was probably "on balance" just leveraged traders/funds being forced out of their trades. The news just happened to hit during a deleveraging cycle and hence the sell down - nothing sinister about it; in the absence of a meaningful buyer, it's just the natural course of trade.
COK Price at posting:
39.0¢ Sentiment: Buy Disclosure: Held