It's usually from too much moisture. It's critical to squeeze as much liquid from the zucchini as possible. A mesh bag, cheesecloth, or clean kitchen towel usually works best. Wring out as much of the water as possible before adding it to your recipe. Zucchini has a ton of water in it that you don't want in your fritter. If you don't have a mesh bag or cheesecloth, you can put the grated zucchini into a fine-meshed sieve and press the zucchini with your hands to let the water fall out of the bottom into the sink or a bowl.