Shields themselves don't come with an added defense bonus. Rather they add to your evasion based your shield skill. How your defence could decrease is a mystery to me, since none of the other weapons increase your defense in any way, at least the actual defense stat. When you say defence, which stat are you talking about? There's three stats. "Defence" which reduces damage of each physical hit by that amount, Defence X which is the number of attacks that you can evade, and Defence % which is your chance to avoid each hit.
