Rhythmic breathing, where inhales follow exhales, is the default for controlling breathing.

“Now, up until now I've said that rhythmic breathing is the default. Pre-Botzinger nucleus controlling rhythmic breathing is the default and that doubling up on inhales and exhales is something that happens when you deliberately take over the action of pre-Botzinger complex. Now, that's true 99% of the time. However, there are certain conditions, such as conditions of heightened state of emotional arousal-- if you think about somebody who's been crying, oftentimes they'll do the double inhale, exhale [INHALES SHAKILY] or triple inhales. Or if somebody is very, very afraid, it's all inhales.”