I just posted this in another question; it is similar to what you are seeing but not exactly the same.
I created a new playlist and then added 1100 songs to it. About 1/2 of them changed the ratings after adding to the playlist. I looked for common factors (cd burn vs itunes store purchase, date added, original rating vs new rating, item above/below...every data point I could think of.
There didn't seem to be any logical formula.
Have you figured this out yet?
I wouldn't say I have figured it out. I found some work arounds such as setting ratings by selecting the '>' next to the song title in the 'Now Playing' window.
Also as far as I can tell the bug only affected other songs that were visible in the current view, so by selecting a specific album it might limit the possible scope of the bug. However, short of reviewing playlist counts after every rating change I can't verify this always works.
I have seen one situation similar to what you report. I added an album to a playlist and every song reverted to 2 and half stars. I'm not even sure how to make half star ratings. But this was limited to one album and didn't affect anything else in the playlist.
How many total songs do you have in your main library. Most of the instability issues like this I've seen from other posts and users are with people with fairly large libraries (10,000 songs or more).
In regards to the 1/2 star ratings, it is my understanding that the 1/2 capability has always existed within iTunes but they have always chosen not to make it obvious/actionable with standard iTunes functionality. There are second party add-ons that will allow you to utilize the hidden functionality and rate songs with ratings in 1/2 star increments but they aren't stock from Apple.