I think Cressida is just a bully, and she sees Pen as vulnerable because of her size, perceived unattractiveness, lack of confidence and barely financially solvent family. Bullying Pen makes her feel powerful.

I think that, like most bullies, she has to the ability to single out the weakest in the room and, for her, Penelope was an easy target. She’s also potentially grown up with the Featherington sisters & seen how the whole family bullies Penelope. In season 1 Lady Cowper and Portia seem to be friends, or at least be close enough that she’s at the Featherington’s house when Marina arrives. It’s possible that Cressida has seen how Prudence and Philippa treated Pen & is mirroring that behaviour.

In Anthony’s book Penelope tells Kate that she accidentally bumped into Cressida their first season and caused her to spill her drink on her dress. Cressida then decided that that was why the gentleman (a confirmed bachelor) didn't propose. So she's targeted Pen ever since.

Cressida is bully and picks on who she thinks is the weakest; she is a “cruel back biter”.

I wonder if Mommy dearest (Araminta) was in love with Edmund and when he choose Violet it set her off. It seems very personal to be just petty jealousy.

Ok, I started typing a response to this that kept expanding in seventeen directions, so apparently I have A LOT of thoughts about this. Do you mind if I repost this later to share my monster thread? 😂

She’s just your classic mean girl preying on the weak. Or at least thats what she thought.

Lady Cowper and Lady Featherington are friends, in S1 Lady Cowper is seen visiting the Featherington house gossiping re: arrival of Marina. So maybe they are childhood acquaintance but Pen is closer to Bridgertons and it seems to me that Cressida also likes to be part of the Bridgerton friend group and like Penelope has also a crush on Colin. and Colin is always warm to Pen. Colin and Cressida were dancing in S2 just before the revelation of Jack F. scamming the ton.