The Blades were cruising to victory 78 minutes in at Oakwell, 3-0 up and with the home fans having turned on their players.
But when Devante Cole's goal was quickly followed by one from Aaron Leya Iseka, what was a stroll of a game for the visitors turned into a nervy finish.
"For some reason we found it a different game," said Jokanovic, whose side had scored twice through Lys Mousset and once from Ben Osborn.
"To be scared to play football is outside of the normal. You can be scared of water or lions but not playing football.
"We should be able to play with more personality.
"It's my sensation when they score the first goal my team is thinking they will score two or three more.
"If you are thinking in this direction it's negative.
"You want to be thinking about being organised and looking to score the fourth or fifth with all respect to Barnsley.
"We gave them oxygen to believe they could find something positive. It's simple but it's not acceptable."
It overshadowed what until then had been a good performance from the Blades, who took the game by the scruff of the neck after a scrappy first half.
"I have to be satisfied with victory," said the Serb. "I must be satisfied because we have three more points but on the other side the first 45 minutes we didn't find enough spaces and enough quality to have a lot of passes but we tried to play behind them without being so successful.
"In the second half we found more spaces behind the Barnsley defenders and scored three early goals.
"We could have scored more but I can be satisfied with this part of the game.
"But I go home really hurt after this game because of what happened in the last 10, 15 minutes was not acceptable."
Jokanovic revealed Mousset's substitution was due to a slight calf problem.