Brighton had missed a first-half penalty and had a goal disallowed just moments later when scorer Neal Maupay was adjudged to have fouled the Palace goalkeeper Jack Butland.
"I think it was a foul," Albion and Belgium star Trossard admitted.
"It's hard to describe. He catches it and maybe lands on Neal as well.
"You never know which way it will go and the referee decided to rule it out."
The Seagulls finally scored a, legal, goal in the 87th minute when in-form Maupay's dangerous low cross was turned into his own net by Joachim Andersen.
Trossard was asked how he felt after the 1-1 draw.
"Mixed feelings," he said. "Great to make the equaliser at the end, as a team.
"We had the best part of the game, where we were in control and should have taken the three points. That's football.
"At the end, we were happy we scored the equaliser."
Trossard said a defeat would have been as painful as last season's home loss against Crystal Palace, when Christian Benteke scored a 95th minute winner completely against the run of play.
"If we had lost this game, it would have been the same feeling as last year," he said.
"We were in control and had the better chances. We did the same today. Luckily we got a point."
On the overall battle with Crystal Palace, Trossard said: "You always feel the rivalry. That's nice about football.
"If you can score for them [the fans], you know what it means.
"For us, it's another point and we'll go again on Tuesday [against Chelsea] It's not long for matchday again but that's what we live for.
"Everyone is ready to play. We are ready to take a result again."