Someone dumped 4 million shares at close forcing the price way...

Someone dumped 4 million shares at close forcing the price way down.
I read the announcement today and at worst it seemed fairly neutral.
I wonder if there is something happening behind the scenes as the company seems to be doing quite well based on what they announce to the ASX.

I normally sell when it hits my stop loss but this time I held and learned my lesson well - currently down 30% from my buy price - an expensive lesson in money management