Were you playing against the computer?

I believe that Soren made it so you could buy the computer out by holding down shift and clicking on the buy stock button. Perhaps that's enabled. We've flipped it on and off a few times and I forget its current state.

-Scott-

It also works on humans in multiplayer. If you can save up the cash quickly enough, it's actually cheaper to do it all at once this way I believe. Because buying someone 10% at a time causes the later stocks to be higher price, and I don't think a straight buyout accounts for that.