The 24-year-old, who had qualified in the top four at every race so far in 2025, lost control at Turn 3 in Q3 and hit the wall, bringing out the sixth and final red flag of a record-breaking session.
He will start ninth in the race, with championship rival Lando Norris only two places ahead after brushing the barriers on his own final run.
“I think I just braked a little bit late,” Piastri told Sky.
“I have not seen any data or anything, but it’s normally what happens when you’re locking your brake, it’s a bit late.
“So yeah, disappointing obviously. I feel like the car was in a good place and yeah, disappointing way to end.”
The McLaren driver said the conditions — with high winds and light rain disrupting running throughout the two-hour session — made grip difficult to judge.
“The first two corners were really good, and then I don’t know if I just tried a bit too much or the rain being around,” he added.
“There were some corners even on the out lap that felt a little bit tricky.
“Ultimately I need to go and have a look, but I think a case of just trying a little bit too hard.”
Asked if he still believed victory was possible, Piastri was realistic about his chances.
“I think the win is ambitious,” he admitted.
“I think we’re going to make progress, the car has been quick this weekend so hopefully we can use that to make some progress.”
Team principal Andrea Stella defended his driver, noting how treacherous the conditions had been.
“There was a lot of wind. It was difficult to put laps together,” Stella said.
“I think Oscar was actually on a very, very strong performance.
“I’ll have to look at the data carefully, but it just seemed like he overdid a bit in braking and we know this place is very unforgiving.”
Stella added that Norris had also been caught out while on a strong lap, costing McLaren the chance of a stronger result.
“To some extent this affected also Lando’s qualifying because he was on a good lap in Q3 and he also touched the wall and he lost a lot of time.
“This can happen in Baku. We will reset, we will repair Oscar’s car.
“It will be a long day today and tomorrow and we look forward to a race where we want to recover the positions.”
Verstappen takes wild Baku pole, Lawson stuns with P3 as Piastri crashes