I think the key indicator here is that you weren’t breathing hard. That makes me think that either you were just having a bad day mentally/physically or maybe your cadence was too low and you just mashed your muscles into oblivion and couldn’t muscle through it any more.
VO2 work should be done with a relatively high cadence in order to put some of the burden on your lungs and make it more repeatable for your legs.
What was your cadence like during your intervals?