After a 1-2-3 first inning, Bumgarner struggled in the second and third, allowing four runs on seven hits and a hit batsman to fall behind 4-0.
The Giants got a run back in the top of the fourth when Buster Posey beat out a single to the pitcher then went to third on Hunter Pence's single and scored on a fielder's choice grounder. The Giants kept their rally going with a Gregor Blanco single and Brandon Crawford's fielder's choice on a screaming grounder stopped by a diving Chris Nelson at third.
That brought Bumgarner to the plate with two on and two out, and the pitcher turned on a 93-mph, 2-1 fastball, driving it 416 feet into the left-field stands for a game-tying three-run shot, his second career roundtripper.
"He got us back in the game with the home run," Bochy said. "That was huge, to answer back and get four. I know he wanted a shot at the win, but we have enough relievers to pick him up."
It was Rockies starter Jhoulys Chacin's last inning, due to the Rockies' 75-pitch limit for starters, but the three-run blast from Bumgarner certainly sealed his early departure.
"I was trying to throw a fastball down and away, and that ball just ran back to the middle," Chacin said. "I know he's a good hitter. In the big leagues, whoever is hitting, if you miss your spot, you make your mistake, they'll take advantage of that. It was a really bad pitch to him."
The relentless Rockies got to Bumgarner again in the fifth when Wilin Rosario bounced a single off third base and Nelson bounced a single off third baseman Pablo Sandoval to set up Matt McBride's run-scoring single that drove Bumgarner from the game.
"I'm all for helping us win, but my job is to go out there and pitch, not to hit," Bumgarner said, disappointed with his performance on the mound. "The ball was up a lot. That didn't work out too good. I didn't have very good command of really any pitch away. I pitch in a lot, but you got to be able to throw away, too, and I wasn't able to do that. It seemed like they were just sitting on one spot."
It was Bumgarner's fourth consecutive outing of allowing four or more runs in less than seven innings and only his third outing under five innings this season.
"He's got a lead, two outs from getting a win, believe me, it's the last thing you want to do is go get the pitcher," Bochy said. "He made a good adjustment. He started getting the ball down better, and his stuff was crisper, but his pitch count was getting up there pretty good."
The San Francisco bullpen was lights-out, striking out eight of the next 10 batters. Dexter Fowler got to second on a two-base throwing error from Crawford to lead off the sixth, and Jordan Pacheco connected for his third hit of the night to represent the only Rockies reaching base against the relief corps through the seventh inning.
The Giants' bats maintained a steady onslaught once Bumgarner tied the game with one swing in the fourth. Brandon Belt plated a pair with a double to left in the fifth, and Angel Pagan and Marco Scutaro each drove in runs as Pagan knocked his 12th triple in the sixth to tie the San Francisco single-season triples record shared by Willie Mays (1960) and Steve Finley (2006). Scutaro brought Pagan home with a sacrifice fly.
"That's a good name to be right next to," Pagan said of Mays, "but at the end of the day, that's not what we're here for. We're here to play hard and win some ballgames."
The rally started with a pinch-hit single to right from Aubrey Huff, 3-for-5 as a pinch-hitter since coming back from the disabled list. Huff started hard for second before stopping in his tracks, turning back to first and diving for the bag, coming up inches short. He would have been out at second by at least 10 yards, but a throw was never made to first.
"He smoked that ball," Bochy said. "Good job of turning that double into a single."
"Like a cat," Huff countered, describing his prowess on the basepaths.
Belt capped the Giants' scoring with a seventh-inning, two-out solo shot into the right-field bullpen, giving San Francisco a vital insurance run before the Rockies scored one in the eighth and two in the ninth.
"I just don't want to be outdone by Madison Bumgarner at all at the plate," Belt said. "It was a three-run dinger, but I think I got four RBIs tonight, so I beat him."