Segwit (BIP141) is active since block 481824 in 24th August. I wonder if it was the BIP91 that made the segwit happen? I mean lets say BIP91 never existed, would segwit be activated in 24th August anyhow.
The direct cause for Segwit's activation is BIP 91 which forced all miners to signal for segwit and that thus activated segwit. However BIP 91 was caused by BIP 148 which threatened to force miners to signal for segwit on August 1st and potentially cause a hard fork. If BIP 91 had not activated, then there may have been two chains where segwit activated/will activate on one (the BIP 148 chain) and a chain where segwit is not activated and may not activate. There may have also been no chain split and all miners followed BIP 148 and activated segwit at around the same time anyways.