I had this happen to me today too! Float went down in a fishy looking seam in the canal, set the hook only to realize that it was a snag, went to snap off the jig and the mainline broke
However for me this rarely happens, normally the leader breaks as I typically run 5ish lbs less breaking strength on my leader than my mainline. I think the odd time your lead gets caught up not your hook. However you said this is happening 90% of the time, which is way too often IMO. As others have said check your eyelets and if you don't know how any fishing shop will check for you.
When fishing jigs etc with short leaders for steelhead I dont like the set up of having your lead dangle off to the side on a piece of surgical tubing, 3-way swivel, etc, I guess it's just a confidence thing. I like the weight to be straight up and down on the mainline and rig mine up the same as Dave C with pencil lead or split shot. The only down side is if your weight gets hung up and doesn't pop free, you are going to have to snap your mainline.
If it were me having this problem I would take the rod to a shop and have them closely examine the eyelets, if they checked out I would put new mainline on. Aside from that just make sure you are not dragging bottom with your weight in other words fishing too deep. With jigs esp you should be 2/3 -3/4ish down in the water colum. If my jig hits bottom I immediately abort the cast, reel in, and shorten up.