The game is published by "Untold Tales," and it looks like all three of their titles are marked down to $4.99 right now. It has to be some sort of ownership discount, but I don't know what's triggering it. Dekudeals shows the current price for each title, but all show as $4.99 via the eShop for me.
https://www.nintendo.com/games/detail/the-hong-kong-massacre-switch/
https://www.nintendo.com/games/detail/beautiful-desolation-switch/
https://www.nintendo.com/games/detail/golf-club-wasteland-switch/
Edit: Untold Tales is taking part in Qubic Games's "Winter Fest" promotion. If you own one of several Qubic Games titles, all of Untold Tales's games are $4.99 each. The promotion appears to last until Dec. 31.
https://qubicgames.com/winterfest/
Owning any of these titles triggers the discount: Pocket Mini Golf, Robonauts, Mana Sparks, REKT, Akane, Dex, Blazing Beaks, Puzzle Book, Space Pioneer, Eyes: The Horror Game.
"Winterfest" runs until Dec. 31st, although the normal sale prices are listed as ending Dec. 22.