Five of the six runs charged to Bonderman came with two two outs.
"He just had trouble getting through innings," manager Eric Wedge said. "All the two-out RBIs early on really killed us. They were aggressive. They're an aggressive-swinging ballclub, and they were just squaring the ball up today and knocking it around all over the place."
It was the second straight outing in which Bonderman gave up six runs, having also done so in 3 1/3 innings in a loss to the Cubs on June 30. Although he lasted five innings on Saturday, the final result was the same. The 30-year-old right-hander gave up seven hits while striking out four and walking five, two of which were intentional.
For the Mariners' offense, it was feast or famine against Reds starter Mat Latos. Although he gave up four runs on six hits, Latos also registered 11 strikeouts in six innings.
Much like on Friday, Seattle took an early lead thanks to a first-inning home run, this time off the bat of Kyle Seager, who connected on a two-run shot following an Endy Chavez single. That marked the Mariners' 21st first-inning homer this season, giving them the most in the Majors.
"It was good to jump on the board early," Seager said. "[Latos] is one of the best pitchers going, obviously, so he's got really good stuff."
Seager drove in another run in the top of the third, when he hit a sacrifice fly to score Brad Miller, who walked and advanced to third on Chavez' second single of the day.
Meanwhile, Bonderman was in good shape through three innings, giving up just one run on a Cesar Izturis base hit in the second.
However, after inducing a Jay Bruce lineout to start the fourth, Bonderman surrendered a double to third baseman Jack Hannahan. Following an Izturis lineout, Bonderman intentionally walked Ryan Hanigan to get to Latos, who entered the game with five hits and two RBIs for the season.
On the first pitch of the at-bat, Latos lined a double to right-center field, scoring both runners. Shin-Soo Choo followed with a single to bring home Latos and give the Reds a 4-3 lead.
For Wedge, that sequence was the game's major turning point.
"For me, that's unacceptable," Bonderman said. "I know he hits every day, but that's still unacceptable. You've got to execute, and I didn't. That's the bottom line."
On the other side, Latos was happy to deliver at the plate after giving up the early runs.
"That was a good time to get a hit," Latos said. "The offense picked me up. Thirteen runs is always good, especially when I put is in a 2-0 hole."
After the Seattle offense squandered a chance in the top of the fifth, Bonderman retired the first two batters he faced in the bottom half of the inning. The Reds then delivered a single, a walk and a double to tack on two more runs and take a 6-3 advantage.
Although the Mariners were able to cut into that deficit thanks to a Dustin Ackely single and an RBI double from Brendan Ryan in the top of the sixth, Carter Capps replaced Bonderman in the bottom of the inning and coughed up three runs.
Capps returned for the seventh and threw a perfect inning. Wedge admitted that his performance in the sixth might have been the result of a little rust, as Capps hadn't pitched since June 25.
"It's been a little while for him," Wedge said. "But that's why I wanted to send him out there for that second inning. I felt like he was a little bit better there. He's got to use all his pitches. He has a great fastball, but he's still learning how to use it."
Seattle mustered one more serious threat in the eighth. Trailing 9-4 and facing Cincinnati lefty Sam LeCure, Mike Zunino singled to open the inning. Ryan and Miller followed with a pair of walks to load the bases, but with two outs, Chavez flew out softly to left field to end the chance.
That capped an 0-for-11 day with runners in scoring position for the Mariners, who left 11 men on base. Seattle collected eight hits, led by two each from Chavez, Seager and Ackley. Miller also notched three walks.
The Reds registered 13 hits, including an 8-for-17 performance with runners in scoring position.
"Today was definitely a tough one," Seager said. "They swung the bats really well today. We had opportunities to score today, got a few runs across early, but weren't able to do much after that."
Despite Saturday's struggles, the Mariners head into Sunday's series finale 3-2 in the first five games of a six-game road swing, with a chance to win each series on the trip.
Jeremy Warnemuende is an associate reporter for MLB.com.