As you drag the bait through stumps and logs, the worm or weight will hang. When the bait is pushed down, the hook becomes exposed and then hung up you are. Most anglers wanted to use a limber rod back when this rig first appeared, so they could feel the fight when they caught a fish. This was probably the cause of more hang ups than anything. The rod you use is the most important part of fishing this rig correctly.
Some people hate worm fishing because they hang up too much.
You need to realize that your bait will be where the fish are and hang ups are just part of it. When setting up the rig I like to use the smallest weight possible as this makes the bait appear more life-like and it won't settle into the bottom sediment as quickly. There are days I like to put a bead between the hook and weight, and I have had days the fish don't want the noise. In the spring, or when pitching the bait, I like to peg the sinker to the top of the worm so the weight won't move. During this time I will use a heavier sinker because I want control over where the bait lands.