Very sorry to hear this.
With the Tac15i you are supposed to load the arrow first, point the bow in a safe direction, and then crank in back.
This is the procedure shown in the video they send with the crossbow.
As a safety precaution, before cranking the bow back I reach into the action with the little finger of my right hand and cock the trigger. Then you can put the safety on before starting to crank the bow.
You then load the string D-loop onto the traveler (the rear pawl) and then lastly the arrow onto the string before you start cranking it back. Then you crank it back gently at the end of the travel until you see and hear the traveler car prongs go click into the detente.
Lastly - I am constantly very aware of the tremendous power stored in this bow. I never get my fingers or hand inside of the string travel path. It's easy to get lazy on stand are hold onto the string, or the crossbow rail from above. This is a mechanical device, and mechanical devices are prone to failure, however unlikely. If the cocking string fails or the dloop fails when your hand is inside the travel path of the string all sorts of bad stuff can happen, as the OP well knows.
Consider that this crossbow stores almost as much energy as a 9X18mm pistol round.
I urge everyone to treat it with the greatest respect, least you get hurt by it if something goes wrong.
I was remind of this the first time I tried to unload the Tac15 in a tree-stand prior to lowering it to the ground. I messed up and somehow discharged the bow. Fortunately I always point it in a safe direction. It buried the arrow into the ground all the way to the fletching. That scared the bajeebies out of me.