Can BIP91 cause a chain split?
The minority of miners that do not support BIP91 (and by extension BIP141), could start rejecting blocks of those who are signaling BIP141, thereby creating a different chain.
  Is that a possibility both in theory and in practice?


Yes that is possible, but unlikely. Since BIP 91 has a majority of the hashrate, I think it is unlikely that there will be a persistent chain fork. Rather what is more likely to happen is that there are short multi-block forks that are then orphaned and reorged out as the majority hash rate chain becomes longer than those forks.