It depends why. It could be something as inane as someone having...

It depends why. It could be something as inane as someone having to sell down to raise some money and accidentally hitting "At market" then the market saying "thanks for that numbskull" and taking full advantage of it.
Which in turn may have some lemmings selling ....following suit.
On the other hand, if it is a reaction to an as yet undisclosed production figure then there needs to be an investigation for insider trading....that said you would assume whoever might have that info would be smart enough to feed slowly into the market at what was a rising market earlier in the day.....
Lets assume it is the former. Fickle thing the market.

564xxl