Kurt,
You may not have done anything wrong. I use doubles to a range of leaders for a range of purposes, including a lot of jigging and reef fishing. If you get totally bricked and have to break off it's common that the middle of the bimini lets go. It's just the weakest link in the whole chain to the hook. Where else would it break?
If the bimini doesn't break then I wonder what I did wrong.
Maybe too much drag plus water pressure? Remember the load at the knot near the fish is lot more than the load at the spool on your reel, and gets worse as the length between the 2 increases and the speed of the fish increases.