What if the bomb blast in which Aastha was killed wasn't a terrorist attack.. but a planned attack by Kiran!! Im just making a guess.. but u never know what can happen in serials. Kiran hates Simran so much, and she even told Simran that "she wont leave her", when Simran and Aastha came to her house after they reunited.
Kiran is the kind of woman that can do anything!! But actually.. maybe that's not possible also because how would Kiran know that Simran and Aastha would be in that bus.. unless she had someone follow them! Well if ever this is true..then she got her revenge because Aastha is dead!
I dont think this theory of mine could be true.. but like I said, anything is possible in serials!!