He did an interview with Louie Vasscher. Basically, he wants the best shot at the NFL, and he doesn't think his fit in NU's new defense puts him in position to be a high draft pick (like many of us originally suspected when he entered the portal).
I thought this was the money quote: "The way the 2020 season went for me, in Coach Hank’s version, I think I was the best version of myself. I didn’t feel the same way this year."
Ouch. He goes on to say nice things about O'Neil, but IMO that quote says it all.
I recommend you read the whole thing, because Joseph has a lot of good things to say and provides more context than me just pulling out this one quote.