The big cases are the ones that the justices care most about, and they are usually badly divided on them. If the justices are unanimous, they can turn out a single opinion in under six weeks that speaks for all of them.
But in a very big case — like Dobbs vs. Jackson Women’s Health Organization, the one deciding the fate of Roe vs. Wade — they are likely to be split. And dissenters will write extensively to explain why the majority is wrong.
The chief justice says he would like to have all the opinions done and ready to go by the end of June, but he has limited control over his independent-minded colleagues.
Knowing that such decisions and dissents will go down in history, they sometimes keep writing up to, and beyond, the deadline.