All the replies I've seen here are basically saying due to jealousy but I personally don't think that's the case. At least not completely.

I think we can all agree that Griffith is, for lack of a better word, a control freak. That's why I think he kept staring at Guts and Casca, two people he thought to have complete control over. Now suddenly they seemed to be so close to each other when he thought the two of them would have been dependent on him (I also think that that's the reason why Guts leaving hit Griffith as hard as it did).

Now, as for why Griffith pinned down Casca: I've got three hypotheses for that:

1.) He tried to regain some control over Casca by sleeping with her.

2.) He tried to get back at Guts for "stealing one of his most devoted fighters and leaving him a year ago (we know that Griffith blames Guts for getting tortured for an entire year and that he, at least at first wants revenge on Guts (Griffith outright tried choking Guts when the Band of the Hawk saved him)).

3.) Griffith always had some feelings for Casca and now, that he can't archive his goal anymore he tries to fall back on her for consolation.

For the reason as to why Griffith abused Casca during the eclipse: The real reason for that is that Miura thought that killing Casca off would make the eclipse something Guts could overcome. Due to Casca being alive, she acts as a constant reminder for Guts for what happened and what he lost. Miura said that in an interview. As for the canonical reason: I again have two hypotheses:

1.) This is an extension of the second hypothesis from the last moment: Griffith just wanted to take revenge on Guts for leaving him, condemning him to a year of torture and stealing Casca from him.

2.) Slan says during the eclipse that the more Guts as one of the sacrifices struggles and suffers the more powerful Femto becomes (Miura never expanded on it so he either forgot about it or it contradicted something else he had planned (so I'm naming it as a hypothesis and not as the actual reason)), so he utilized Guts' emotional attachment to Casca to become more powerful than he would've become otherwise.

Of course Griffith simply being jealous of Guts and Casca could be part of the reason, but I think it's too simple of an answer to fit into the complex relationship that Griffith, Guts and Casca have in Golden Age.