Two key points:
1) There is a clear seasonal pattern for house prices.
2) The surge in distressed sales during the housing bust distorted the seasonal pattern.
For in depth description of these issues, see Jed Kolko’s article from 2014 “Let’s Improve, Not Ignore, Seasonal Adjustment of Housing Data”
Note: I was one of several people to question the change in the seasonal factor (here is a post in 2009) – and this led to S&P Case-Shiller questioning the seasonal factor too (from April 2010). I still use the seasonal factor (I think it is better than using the NSA data).
This graph shows the month-to-month change in the NSA Case-Shiller National index since 1987 (through July 2022). The seasonal pattern was smaller back in the ’90s and early ’00s and increased once the bubble burst.
The seasonal swings declined following the bust, however the recent price surge changed the month-over-month pattern.
The second graph shows the seasonal factors for the Case-Shiller National index since 1987. The factors started to change near the peak of the bubble, and really increased during the bust since normal sales followed the regular seasonal pattern – and distressed sales happened all year.