You're on the right track. The strength of the electric field around a charged, curved metal surface is inversely proportional to the radius of the curve. Sharp points have a very small radius, so they generate a large electric field, which can easily ionize air and thus cause arcing.

So spoons are okay, as they normally have no sharp points. Forks can cause problems, though.

Actually there's another issue here. If you stick an unopened can of food in the microwave, the microwaves can't reach the food; you get the same effect as if you ran the oven empty, which is that the field becomes very intense and risks burning out the magnetron or other components. The oven is designed to have its output absorbed by something, not just reflected back.