Relievers Liam Hendriks, Santiago Casilla and Sean Doolittle gave up two runs apiece in just two innings, allowing the Astros to rally for an unconventional win that negated what had, up to that point, been a solid day for Oakland.
The A's clubbed two home runs, including Khris Davis's sixth in 12 games, and led 5-0 before the wheels came off.
"What is it, eight walks and two hit batters? We just couldn't finish it off," A's manager Bob Melvin said. "We played so well and did everything right for five innings and couldn't play any worse and did everything wrong [after that]."
Houston scored twice off Manaea without getting a hit in the sixth inning, then pounced on Oakland's bullpen after reliever Dull's one clean inning.
Hendriks lasted just one-third of an inning and gave up three hits and two runs before departing for Casilla. Casilla, the former Giants closer, lasted a full inning but allowed two runs, while Doolittle followed and also gave up a pair of runs while retiring just two batters.
"Hopefully we got it out of our system," said Doolittle. "There were some guys coming into tough spots with guys on base, but we have to be better coming into games in those situations. We talk a lot about we're a unit and we're a cohesive group down there and we're only going to be as good as the sum of our parts.
"It's frustrating not being able to pick your teammates up and not be able to leave those guys on base. Dull makes it look so easy, but sometimes it's tough."
The bullpen's collapse came not long after Manaea's day went south.
Oakland's second-year pitcher cruised through the first five innings, but lost control in the sixth when he walked the first three batters on 15 pitches. Manaea's final eight pitches and 12 of his last 14 were balls.
"Today was a tough one" Manaea said. "Just completely on me because everybody was sitting around waiting. I had long innings, just putting the bullpen in a bad situation. For me it's really a mental thing. Nothing anybody else could do except for me."
Michael Wagaman is a contributor to MLB.com and covered the Athletics on Saturday. This story was not subject to the approval of Major League Baseball or its clubs.