Nandhini wanted to protect Manik's Music
I will try to explain Nandhini's Point of view regarding Yesterday episodes events...
Just the previous night after the painful vinegar torture, Manik and Nandhini had shared a meaningful moment , where Manik actually opened up to her and Nandhini got through to him.She conveyed to him hurting himself by hurting her was not the correct way to do things, he should instead search for his way back to his Smile , search for a way she could help him get it back.
In the Morning Nandhini saw Manik trying to bulldoze his way into the classroom and concert of Pandit, the very person he detested earlier and become her partner , the person whose hurt he is trying to deal with and who he is Angry with.
If a Normal Person didnt know that Manik was doing this for protecting her, they would have clearly seen this as she saw , as one more of his attempts to make her Pay , Do something to punish her...maybe even spoil the concert.
If it would have been just pain for her ...Nandhini would have gladly accepted his action, infact would have fought on his behalf to get him in, she would have taken all the Pain and Hurt happily...but it was not restricted to her alone...if Manik used the concert to get back at her... if in his anger he acted out in the concert, his actions would damage his career as a Musician.
Already he had walked out of an Audition for her, if he now did something similar in the concert or worse...his career in Music would be affected badly, which she was her worst nightmare.
Her fear that she would be the reason for damage to his career in Music...she already blamed herself during the Hospital track and then when he walked out of the Audition...She couldn't let this happen at any cost...hence she did all she could do to stop it, first tried to make him understand and then tried by cutting the guitar chord...
Remember she is the same Nandhini Murthy who kidnapped him , hid in his Car among luggage to Protect him...cutting a Guitar Chord is nothing in front of it...she would have gone to even greater length to protect his career in Music...That is Nandhini Murthy...!!!
She does not back away from taking his Hurt and Pain... never ! and she cherishes his presence around her like a miracle in her life...if someone thinks she wants him away so that she could be alone with Pandit or she fears his "I will make you pay"...then they have not even started to get to know Nandhini Murthy...To actually see her and get to know her...a Clean slate with no pre-concieved bias or notions is needed.