There could be a couple of reasons. You may be putting too much water and/or dye into a small area and it is actually flooding across the gutta. But most likely the gutta is too thin in sections. I find that if you work too quickly when drawing with gutta, often it barely sits on the surface and does not penetrate to the back of the silk; the dye can jump under the gutta line and cause a bleed. You have to learn to keep the flow of the gutta even and press firmly enough (without damaging the silk); but above all slow down and this will help you keep the pressure and flow correct. The gutta must completely penetrate the silk. When a painting is complete, the back should look the same as the front.