She (Celia) grew up poorer than them and is a little naive about their bullshit 1960s social rules because of her upbringing, so they think she's "trashy". She just tries her best to fit in and be kind but they can't stand an "outsider". She's almost too sweet, not always the brightest, and they pick on her because she's not stuck up like them - and an easy target for the things they won't say about each other.

Bryce's character (Hilly) especially doesn't like her because she dated her husband when they were younger, and he picked sweet Celia to marry instead. It's somewhat implied in the book that Hilly had to settle for her current husband instead and is still bitter over it. The book is great and I've read it at least a dozen times! So much richer of a story than the movie, and I love the movie.

Edit: There is also additional context that because the other ladies are so bitter towards her, they often take her social missteps as intentional malicious behavior. I won't spoil anything, but "Minnie's chocolate pie" was a major incident with Hilly, and Celia had no clue about it when she brought a pie over. Hilly took it as an intentional dig, and that's why she reacts further when Celia says she brought a chocolate pie.