Bayern, Bundesliga champions in the previous 11 seasons, finished third behind Bayer Leverkusen and Stuttgart, and will go without domestic or European silverware for the first time in more than a decade.
"We started really well but then we had a string of string of individual mistakes," Tuchel said in a short press conference.
The coach is leaving Bayern after just over a year in charge after the club announced his departure in February.
There had been talks last week to potentially reverse that decision, but no agreement was reached.
"In the second half we played well for 15 minutes but then it was again a series of individual mistakes and loss of possession," Tuchel said. "We gave away a two-goal lead for a completely unnecessary and bitter defeat."
Bayern, despite a string of injury absences, were 2-0 up by the sixth minute but Hoffenheim scored four times including an Andrej Kramaric hat-trick in the second half to turn the game around.
"It happens far too often, like that. It happened in Stuttgart where we conceded two late goals (for a 3-1 defeat). In Heidenheim the same (for a 3-2 loss), here the same. It just happens too often," Tuchel said.
"I have an explanation for that but it is not for the public," he said.
Bayern's Aleksandar Pavlovic, who is part of Germany's preliminary Euro 2024 squad, went off injured early in the match and will under go further tests.
"He twisted his ankle. We will have to wait and see what the checks show," Tuchel said.
