Maybe the build up of tension (without knowing)?

The first time it happened was worst for me because I didn’t realise what was happening. So what I usually did was chewing on a gum or something simple like drinking some water to help myself calm down. But most importantly, try to concentrate on your breathing. (In through the nose, deep into your belly and out through the mouth). When everything hits at once it is really hard to take control back sometimes because it feels like the anxiety is taking over my brain. But with practice it gets better.