I agree that they are usually triggered by something. Some type of anxiety. I haven't experienced a full panic attack, but what I went through recently was nasty as well. It was triggered by some type of anxiety which caused a shortness of breath, then I felt like I was suffocating, on the verge of passing out and like death was imminent. When you are in that state of mind it can be quite scary. All you want to do is just escape somewhere where you'll be safe, nothing else matters.
Don't cry, don't regret and don't blame
Weak find the whip, willing find freedom
Towards the sun, carry your name
In warm hands you are given
Ask the wind for the way
Uncertainty's gone, your path will unravel
Accept all as it is and do not blame
God or the Devil
~Born to Live - Mavrik~