I really shouldn't be too surprised to learn he's being charged with insider trading.
I won't pretend to know anything about this case other than the extremely limited facts that have come forward to this point, but it sounds like he was kinda stuck:
But the SEC is saying that an investment bank recommended to the head of Mamma.com that they invite Mark Cuban to be a private investor in their PIPE investment vehicle.
A PIPE is a Private Investment in a Public Entity. It's a way for a public company to raise money -- fair or not, the view is that a lot of them are done when a company is getting a little desperate for cash, which tends to make the market a little nervous.
The CEO allegedly reached Cuban, told him the call was confidential, and then told him about the PIPE. Cuban didn't like it, and allegedly told the CEO that he was screwed, and now would not be allowed to sell. And, according to the rules of stock trading, that was exactly right. He had insider information and could not sell. But then the record shows he did sell.
Got that? He's a shareholder in this company, right? They call him up and say "You totally can't tell anyone, but we want to you to be a part of our 'special investment vehicle', which is a PIPE loan."
(PIPE financing is when a company really wants money and is willing to sell additional shares at a discount relative to market price. It's kinda desperate, and it dilutes the value of the existing investors' shares. It's generally not a good sign.)
So Cuban is all "oh, snap, this company is borked whether I give them money or not." He sells the stock. That's where the insider trading happened.
Of course, the SEC exists to eliminate or at least reduce the impact of these asymmetric information flows, but it's hard not to sympathize with the guy. They essentially handcuffed Cuban, daring him to either pony up some money for additional shares, or lose money when the company collapsed.