@givemersspls well I know that what you are saying does make sense. But try putting yourself into her shoes...
A lone girl in a lonely place at late night where they throw there trash and suddenly she spots a guy who was trying to hide behind the trash dressed all shady along with mask and cap.
Any girl would creep out. Its a human nature to freak out... if it were a guy there he would stand up and ask the questions and might listen to Sunjoon's reasoning but woman are generally more cautious and vulnerable. Hence she freaked out.
Also the shady guy acknowledged that he knew her... which is why the mother panicked even more and thought that guy was after her from the very start and was waiting for her behind the trash can.
When asked to reveal himself and take off the mask... he refused and which certainly soldified her doubts about him being some kinda pervert/murderer after her.
The guy told that he will show her. (Didn't specified that he was talking about face) the mother mistook him some kinda pervert flasher and threw that trashbag which was only the thing she had.
While she called the police, he grabbed her to stop her but by then the Hyo-chan had returned and saw him. Seeing some shady guy grabbing his mom's arm as well as he saw mother's panicking face he concluded he was some kinda pervert attacking his mom and charged without thinking for a second.
That's the most of it.
The attacks on woman such as this are increasing and it won't be surprising that any girl would do such things. Try thinking more humanly instead just judging by what's wrong and right.
I hope this clears some of your doubts.