BIP 91 locks in, but it could go wrong

Even though more than 80% of miners have signalled for BIP91 activation, this does not mean that these miners will signal bit1 for Segwit. At the moment the proportion of miners signalling for bit1 is far lower than that achieved for BIP91.

What this means ?

Most users accepting bitcoin payment, don't recognize
BIP91 or it's btc 4 signalling, meaning they'll accept blocks with or without bit1.

If miners decide not bit1, even after BIP91, they'll continue to mine blocks and if they are a majority they'll end up mining the most valid chain. This will result in minority miners who enforced BIP91 to be forked off the network.

Miners have made statements to activate Segwit, but this is still just a statement and they could change there minds.