It’s a battle between barely-legal bump-fire stocks. Slide Fire Solutions, makers of a patented bump-fire floating stock, are suing Bump Fire Systems, makers of the very similar (and much cheaper) AR15 BFSystems stock. Kevin’s Tech Ramblings covers the issue in greater detail.
While the Slide Fire product retails for around $300, the Bump Fire stock costs about a third of that. I am not the person to ask about patent law, but it does seem as though the BFS product is very similar to the Slide Fire stock.
Thanks to Kevin for the tip.