Act III Viper Napuatzi Boss Fight - Locking Arena Size

During the Viper Napuatzi boss fight, if you stun her during the Meteor Shower ability and do enough damage to the boss to push her into the Xopec's Spear and then freeze her real quick the arena size gets locked. The guard's shield stay up, they can still thrust their spears, however they do not step in afterwards or any subsequent Xopec's Spears.

I didn't get a screenshot during the boss fight but took one after the fact showing the position the guard "locks" into.