Perhaps it was an accident during a training session. Kurenai could have been teaching Naruto a new jutsu or combat technique, and things got out of hand. Naruto might have made a wrong move, and in an attempt to correct him or protect herself, Kurenai accidentally hurt him. It could also be that there was some sort of emotional turmoil within Kurenai. Maybe she had received some bad news or was dealing with some unresolved issues, and Naruto just happened to be in the wrong place at the wrong time and became the target of her outburst.
One possible reason could be a misunderstanding. Maybe Naruto was misidentified as a threat in a tense situation. For example, if there was some sort of dark jutsu at play that clouded Kurenai's judgment and made Naruto seem like an enemy. Another reason might be that Kurenai was under some sort of mind - control. There are many villains in the Naruto universe who are capable of such things. If she was being controlled, she might unknowingly attack Naruto.
It could be related to a mission. If the mission required Kurenai to sacrifice someone for the greater good and Naruto was the most expendable in her eyes at that moment (although this is very unlikely given her character). Or maybe Naruto disobeyed a direct order from Kurenai during a mission which put the whole team in danger, and as a result, she had to take drastic measures to stop him which led to him getting hurt.
The best chapter pointer here is Chapter 197, "Dear Brother". It opens with Seeing that no one cared about what she said, Su Yinwan immediately felt... and closes on Wen Jiaojiao, aren't you underestimating me?.
Chapter 193 is the strongest chapter-level answer because it starts from Jiang Shixu's eyes lit up when he heard there was a gift. and follows through on Jiang Cheng was a prime example.. The payoff is that the chapter closes with [Jiang Li is so disgusting, exploiting the game loophole like that.
If you want the clearest chapter reference, go to Chapter 172. Early on, Not only Shu Yan, but the entire internet was shocked when they saw Su...; later, the chapter leans on Damn, what brand of make-up is Su Yinwan using?.
The best chapter pointer here is Chapter 193, "Do You Really Think the Audience is Blind?". It opens with Jiang Shixu's eyes lit up when he heard there was a gift. and closes on Jiang Cheng was a prime example..
Look to Chapter 220 for the clearest answer. It begins with I know everyone has many questions, and I'm here to answer them one by..., keeps building through Actually, from the very beginning of the seed selection phase, we designed..., and lands on However, Su Yinwan and Shang Shaojing weren't.
The best chapter pointer here is Chapter 181, "Are All the Weibo Followers Police?". It opens with Xiang Hao rushed to the police station as soon as he saw the trending... and closes on Let's not be too hard on a young girl.] However, these defiant words only....
Look to Chapter 201 for the clearest answer. It begins with The third time she passed by happened to be when she was rushing back..., keeps building through Upon hearing Jiang Li's words, Zhao Lang couldn't help but laugh., and lands on Indeed, I'm not well-received." "Ahhhh, these two are going.
The best chapter pointer here is Chapter 184, "This is Cheating!!!". It opens with Jiang Li can finish the competition alone??? and closes on Slow down, I'm so scared." "Shang Shaojing, why do you keep pulling on me?.
Chapter 202 is the strongest chapter-level answer because it starts from Jiang Li couldn't believe it, "Just to take a picture?" "Yes, just to take... and follows through on Give me the card." She stretched out her hand expressionlessly..
The best chapter pointer here is Chapter 187, "I Really Need an Apology". It opens with During the competition, she passed by him numerous times, unfazed by... and closes on Perhaps it was a pig that tripped you; they are quite aggressive, after....