Boss fight stages triggering on them being stunned

My monk alt just did the Normal Act 3 Viper Napuatzi fight. Most surprisingly she never reduced the size of the arena. My main character was busy stunning her, as did my 0.1 warrior. They got the cramped arena treatment. My monk instead froze her. The fight was trivial due to the wide space and lack of phalanx poison stabs. I doubt it's because I have a twink weapon that damaged her fast.

The boss fight in the Temple of Kamasa had the goldsmith running back into the lava even though I didn't stun with my monk. So why is Viper waiting to be stunned before closing the vice?