It's a small detail that is hard too see in the chaos of that Team Fight, but right after Cassio did her engage, Tahm Kench used his W for a chain CC on Kalista to kill her before she could leave the stun and that would win them the fight and probably the game right after.

But Doran instantly reacted to that and was able to stop Peter from using his W and zoned both TK and Caitlyn from joining the TF so T1 could win the fight.