In 1x08 Jason tells Spencer that Ali told him about the fire, how Spencer said they should teach Toby a lesson, and got the other girls to back her up, even threatening Toby if he told the truth.
Yes, we did see Ali do all that but just because she did it doesn’t mean she was the mastermind behind it.
I posted a while back that blinding Jenna wasn’t the type of arsenal Alison used. Alison was a psychological bully not a physical one. She didn’t just manipulate people and then go “hey I should blow someone up, that will be exciting.”
I think Spencer found out what Jenna was doing to Toby and she wanted to teach her a lesson. When they walk over to Toby’s house Alison asks Spencer “You’ve got the lighter right Spencer” and then after arguing Alison declares “give me the lighter” and Spencer hands it over.
Why wouldn’t Spencer do it herself?
She manipulated Ali to do it so she could get rid of her. She didn’t know Ali would make Toby take the fall.
Later on when Spencer and Toby get together. Toby tells Spencer that Jenna is afraid of her.
In 3x09 Jenna remarks to Noel “I will not let Spencer Hastings bully me, again”