How can a mail be deleted just by swiping left?

I was to flag that very important mail sent to me from my bank. So, I swiped the mail title left impatiently with a big big swipe. It's gone? What just happened? Anyway, I rushed to the Trash to recover it. I was a bit nervous then. I was going to swipe it really slow this time. I was so coutious swiping again that as I saw that gray, orange, and red came up I released my finger right away. Well, this time I was overcautious that the title bounced back right. Well, swipe more over left then. A big more over left, the bloody red bar with the trash can icon bled all the way left. And, instead of swiping back right, I released my thumb as if I got a electric shock. Again, the mail disappeared! That was really no good than walking on the wire.