Captain Marquinhos had headed the French champion ahead in a dominant first half on Wednesday but the hosts eventually lost the first leg 2-1 after capitulating in the second half.
City, who were poor in the opening period, wrestled the tie back through Kevin De Bruyne’s fortunate goal and a free-kick from Riyad Mahrez that somehow squirmed through the wall.
PSG also had midfielder Idrissa Gueye sent off after a reckless lunge on Ilkay Gundogan in the 77th minute.
“I must say, in the second half, PSG were protective and not progressive anymore,” Wenger said, appearing as a pundit on beIN SPORTS. “Is it because they were not physically capable?
“After 1-1, they collapsed completely. They became emotional and aggressive in a stupid way.”
Both sides, heavily backed by wealthy owners, are fighting for their first Champions League title.
City is also flying domestically and looks certain to be crowned Premier League champion in the coming weeks, while PSG are surprisingly facing strong competition in an intriguing Ligue 1 title race.
“What was interesting is that you could see that when it was 1-1, PSG mentally collapsed,” Wenger added, pointing to PSG’s patchy domestic form.
“This is the consequence of the team losing eight games in the league. This has an impact.
“When it’s 1-1, the team knows they have lost many games, they carry that. It’s not like the season before, when they were unbeatable in the league.
“You could see the confidence dropped considerably and the mental impact when Man City equalized, for the semifinal of a Champions League, was too big.”
Game of two halves
PSG manager Mauricio Pochettino admitted his team struggled in the second-half as Manchester City began imposing itself onto the game.
The counterattack, used so brilliantly by PSG in the quarterfinals against Bayern Munich, was snuffed out by City’s quality on and off the ball.
“There were two different halves. In the first half, we did well. We created chances and deserved the lead, but in the second half, they were better than us,” Pochettino said after the match.
“The two goals were accidents, but they created more than us.”
Both sides will be aware the tie is far from over ahead of the return leg in Manchester on May 4.
The French side will need the likes of Neymar and Kylian Mbappe to improve on their performances, with both attackers very quiet for much of the match.
Mbappe, considered to be one of the best attackers in world football, failed to record a single shot on target in the entire game.