Hwang Sun-Hong's men lost just six times in winning the title last year, but have started this campaign poorly and are the only side in the division without a point to their name after two rounds of action.
After last week's 1-0 loss at home to Ulsan, they went down 3-1 at Busan I'Park on Saturday to slip to the bottom of the table.
All was looking well for the visitors when they took the lead just after the hour through Lee Myeong-Ju, but two goals in the next 11 minutes from Lim Sang-Hyub turned the match around.
Yang Dong-Hyun added a third eight minutes from time to seal the victory as the hosts registered their first points of the season.
Jeonbuk Motors and Ulsan have enjoyed starkly contrasting starts to Pohang, boasting maximum returns from their first two games.
Ulsan followed up last week's win over the champions with a comfortable 3-0 success over Gyeongnam on Sunday, all three goals coming in a devastating eight-minute spell midway through the second half.
Kim Chi-Gon opened the scoring in the 63rd minute, before Kim Shin-Wook and an own goal from Woo Ju-Sung put the game beyond the visitors.
They share an identical record with Jeonbuk, who won 1-0 at Incheon United thanks to Hyuk Jung's 74th-minute winner.
Suwon Bluewings are two points behind the leading pair after a dramatic finish to their 2-2 draw with Sangju Sangmu.
Hosts Suwon went in front 17 minutes from time through Bae Ki-Jong, but the visitors were level eight minutes later thanks to Kim Dong-Chan's strike.
Kim looked to have snatched all three points when he netted a second in the third minute of stoppage time, but Bae replied with a second goal of his own two minutes later to salvage a point.
Elsewhere, Jeju United won 2-1 at Jeonnam Dragons, while the game between Seongnam and Seoul ended goalless.
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