I can tell you why it happens but... Im not sure if it'll help or not. The trouble is, your nervous system is composed of several different parts. On one level is the one you're used to -- the one you do basically everything you think about with. But theres also a part called the Sympathetic nervous system. This part of your nervous system can't examine the outside world, and interprets things only indirectly through your senses.
Something happens. Something small.. maybe a smell, a feeling, a thought.. .something that maeks you nervous. When yo uget nervous, your sympathetic nervous system considers that to be fear. And it considers anything you're afraid of to be a threat. In response to this false threat, it prepares your flight-or-fight response, most notably by releasing adrenaline. This happens to everyone, but for most people it ends just as fast as it began.
But with panic and anxiety disorders, that feeling itself has become associated with fear. So the adrenaline release causes fear, which your sympathetic nervous system considers a threat, and in response to this threat it prepares your body to fight by releasing adrenaline. Your heart goes faster, your breathing gets shallow and rapid, which scare you. Your sympathetic nervous system interpets your fear as confirmation that you really are in danger, and releases adrenaline in order to prepare you to confront that threat.
Its a positive feedback loop. Triggers itself. And once it does its nearly impossible to stop.
This is why panic attacks tend to spike, reduce and spike again in ten-minute cycles; the amount of time needed to replenish adrenaline.
And that's what happens. What I wrote may seem a bit misleading, since you don't remember doing all that. Thign is, all that happens in a matter of seconds. And by the time its triggered, you're so afraid that it just keeps going on its own.