It seems some of the metadata was changed, accidentallyor not.
Easiest way to recombine them is to go into the iTunes song list, select all the albums and hit cmd-i. iTunes will ask if you want to change the data for multiple items. Click yes. Then fill in the album title, disc x of x (usually 1 of 1), year and number of tracks (blank of x). If all those match, iTunes should consider all the songs as belonging to the same album.
Example, I downloaded a couple Lady Gaga songs from the same album from different sources (Rhapsody and Amazon). One place had the artist as Lady "GaGa" and the other as Lady "Gaga." Just that slight change in capitalization confused iTunes. Making them have consistent capitalization got them placed as the same album.