I haven't played this game a great deal anyway

I have a level 23 swordmaster and level 22 renown. I've been doing mostly BOs and Keeps. I've been wondering how do you get a better roll on the chest prizes?
People have told me different things. I rarely get a prize and when I do - it is usually the lesser bag of holding.

Any tips or is this random?

the roll is random

random roll between 1 to 1000, and then additional roll points for how much you contributed in the kill, damage/healing done that is.

Actually it's not only that but movement is caculated aswell.

The more you move the "better" you preform .