What happened

Shares of Dedham, Mass.-based electric power provider Atlantic Power (NYSE:AT) plunged 11.5% to close at $2.31 per share in Wednesday trading -- apparently for no good reason.

The only news that emerged today was of the positive variety: A Form 4 filing with the Securities and Exchange Commission confirmed that Atlantic's executive vice president of commercial development bought two batches of 10,000 shares each of Atlantic Power stock (an "insider purchase") for prices of $2.35 per share and $2.38 per share, respectively. Both prices were above where Atlantic Power stock ended the day.

Two cartoon characters confused by a falling stock chart

Image source: Getty Images.

So what

That sounds like positive news -- an executive putting his own money on the line to expand his investment in the stock. And yesterday's news was no worse: A press release announced an upcoming sale, in May 2022, of Atlantic's Manchief power plant to a current customer for $45.2 million.

As Atlantic Power explained in the release, it will continue operating Manchief for the next three years, and continue "to realize the cash flows for the remaining PPA [power purchase agreement] term" before ultimately selling the plant for cash, which Atlantic will apply to "support continued debt reduction."

Now what

So why is Atlantic Power down after two days of nothing but apparently positive news? I honestly cannot say.

Perhaps the answer is as simple as this: Atlantic Power is a penny stock, trading for just a couple of bucks a share, and has a market capitalization of only $255 million, making it subject to large percentage moves on relatively tiny (in this case, just $0.30) changes in absolute stock price.

But that's just a guess. And your guess is probably as good as mine.