.... no, actually the game is over a year old and it's quite common practice for a massive Christmas sale

many big games will get a 75% reduction and a Steam promotion. This is common in the gaming industry.
The Adventure Pack just came out a week ago so there's not an option for that to go on sale. Spooky is relatively new so a smaller sale price there; still all good numbers.
There's nothing to worry about. At all. A lot of people will go past their Steam wishlist and buy games for low prices - Christmas sale is an annual thing and arguably where a lot of companies make a lot of money, hence the big reductions. Some players specifically wait for this time of year to buy popular games at cheap prices.
Hope this helps,
[heart]
Bo