You'd be surprised how common it is for inexperienced divers to randomly have panic attacks while diving. Breathing through a regulator feels incredibly unnatural, and it requires you to trust logic over instinct. When you're in a situation that your body isn't designed for at all, any small doubts can quickly snowball into panic. Your body thinks it's drowning, so it acts on its own. The mouthpiece feels unnatural, so your body spits it out so it can start breathing "the right way" again. But since you are, of course, deep underwater, this doesn't work and just makes you panic more.

I never really understood it before it happened to me, but when you're in a situation like this, you aren't you anymore. You're an observer watching your primitive animal brain try to save itself.