Well I know reason #1
So going with the theory that Stiles’ dad dies.
What if Lydia shows up at Stiles’ house to yell at him for something that happened during the episode and he’s in his room and he just found out his dad died, he’s in shock and when Lydia starts yelling at him, he just takes it. And then Scott calls her to tell her what happened. And she’s so shocked that she just yelled at Stiles and his dad just died, and Stiles just hugs her.