Talking with the Los Angeles Times, Carroll said his pending move wasn't in reaction to possible sanctions the Trojans' program faces as a result of an ongoing investigation over whether players may have received improper benefits.
"Not in any way," Carroll told the newspaper. "Because I know where we stand. It's just a process we have to go through. We know we've fought hard to do right."
Carroll, a longtime coach and coordinator in the NFL before joining USC in 2000, said he had for a while "given up" on a return to the pro ranks.
"But it came out of nowhere," he told the Times.
Carroll had earlier reached an agreement in principle with the Seahawks, multiple NFL sources confirmed to ESPN NFL Insider Adam Schefter on Saturday morning.
"I've given everything I've had," he said. "There was never going to be a good time."
Seahawks chief executive officer Tod Leiweke was in Los Angeles on Sunday to finalize the deal, the Times had reported.
The newspaper also reported that USC players received text messages Carroll would resign. A USC spokesperson told ESPN.com on Sunday night that the school had nothing official to announce.
Carroll had already planned a team meeting for Monday afternoon, on the day the Trojans return to class after the winter break. A team spokesman said that meeting is still on.
"By far the most difficult thing is leaving the young guys that just came into the program and just started," Carroll said in the Times interview.
Carroll has also started to form the staff he will work with in Seattle, sources indicated.
The Chicago Bears have been informed that Jeremy Bates, who was scheduled to interview for their offensive coordinator's job, has decided to accept the same job with Carroll and the Seahawks, two sources told ESPN senior NFL analyst Chris Mortensen on Sunday night.
A source said Bates was tempted to be reunited with Bears quarterback Jay Cutler, but the long-term security with Carroll was a major factor in his decision to join the Seahawks rather than the Bears. Bates was Cutler's offensive coordinator with the Broncos in 2008.
The Bears' options for the position now include Mike Martz, Jedd Fisch and Bill Musgrave, league sources told Schefter.
A source close to USC linebackers coach Ken Norton Jr. confirmed to ESPN's Shelley Smith that Carroll will be taking Norton to Seattle to fill the same role with the Seahawks.
Norton recruited Lakewood (Calif.) High School quarterback Jesse Scroggins, who has already committed to USC but is said to be waiting to see what happens with Carroll.
Current Trojans said they had not heard the decision yet from Carroll, but heard about a Los Angeles Daily News report on Sunday saying Carroll had resigned.
"Wow. Well, I'm not surprised," junior running back Marc Tyler said to ESPNLosAngeles.com's Ramona Shelburne. "I think by now most guys wanted it to hurry up and happen so we can see who is going to coach us next."
Sophomore defensive end Malik Jackson said while he was a little surprised, he understands why Carroll is leaving.
"Wow, I can't believe we're one of those teams looking for a new coach," Jackson told Shelburne. "I'd heard about it all weekend, but I'm still kind of surprised.
"I don't blame him though. We all have to make decisions in our lives. If I could've said something that'd make him come back, I would've. But I'm not mad at him. He has to do what he thinks is best for himself. Everybody does."
A source on the USC coaching staff told ESPN that before the news broke, there was a meeting set for Monday at 5 p.m. ET to prepare for the offseason.
Now it looks "like that meeting will be Pete's last," the source said.
The source said at least one assistant coach has been contacted by another team, but many others haven't been contacted because "their status is up in the air."
Speculation concerning Carroll's replacement has Washington Huskies coach Steve Sarkisian high on the Trojans' wish list.
But Sarkisian said Monday in a text message to The Associated Press that USC has not contacted him.
On Friday, Sarkisian said with a laugh that it "would be fun" to join Carroll in being head coaches in the same city.
Oregon State's Mike Riley fell off the list of possible successors when it was announced Sunday night that he had signed a three-year extension through at least 2019.
Instead, Riley, who was a finalist for the Trojans' job in 2000, will stay in his hometown.
"Due to the fact there is a lot of speculation on my future, I want to make it known that I'm very excited to be coaching at Oregon State University and I anticipate doing so for a long time," Riley said in a statement released by the school.
Sources close to Riley said he was intrigued by the USC job, but loved coaching in Corvallis and the idea that he could finish his career there.
The opportunity is unique for Carroll. The Seahawks do not have a GM in place, so he could have authority over football matters like he had at USC, and far more than he would have had filling any of the NFL coaching openings to which he's been connected in recent winters.
And this was perhaps the best time to leave USC since he arrived in 2001.
USC's string of seven consecutive Pac-10 titles ended with four losses in the just-completed season. And the school has been under several years of NCAA scrutiny for alleged improprieties in both Carroll's team and athletic director Mike Garrett's beleaguered department.
When receiver Damian Williams announced he's entering the NFL a year early, the news release of his departure Friday night didn't include a quote from Carroll, who often lavishes praise on his early entry candidates.
His silence on Williams was good news for the Seahawks, who could use some.
Information from ESPN NFL Insider Adam Schefter, ESPN's Shelley Smith, ESPNLosAngeles.com's Ramona Shelburne and The Associated Press was used in this report.