"Maybe he was expecting me to go up the seam," Alonso said.
He didn't. Once he had the ball, all Alonso needed to do was outrun Williams for the score.
"I mean, that's my first touchdown on defense ever, like high school, college," he said. "I felt good."
The Dolphins (5-4) intercepted Rivers four times in a 13-minute span in the fourth quarter and won their fourth straight game.
With the score tied and the Chargers (4-6) on the Miami 42, Dolphins coach Adam Gase said he was thinking through some timeout scenarios "and looked up and saw his hands on the ball. He's a very instinctive player and I know a couple of times he gambles and loses but a lot of times he gambles and he's right."
Miami quarterback Ryan Tannehill said he saw Alonso "buzz out right before the snap and as soon as Phil started looking that way, I was happy he didn't see him."
Rivers tied his career high with the four picks.
"Obviously it can't happen down in there," said Rivers, who helped the Chargers to a 10-0 lead midway through the second quarter. "There were two of them when we were fixing to score, and then obviously the last one there with a chance to win the game with a field goal. The guy made a great play."
Two plays into the ensuing drive, Rivers was intercepted again, by Tony Lippett, his second of the game.
"Too many missed opportunities with the chance to take the lead a few times. I didn't get it done," Rivers said.
Rivers threw three touchdown passes to move past John Elway for eighth place on the career list with 301.
Jay Ajayi ran for 79 yards, ending his streak of three straight 100-yard games, but he had big runs to set up two touchdowns by Williams.
Copyright 2016 by The Associated Press