When casting a fishing rod for the first few ticks the line connecting the rod and the hook is pointing away from (0|0|0).
The following is based on decompiled version of Minecraft 1.8 using MCP. All method and class names are the names used in the decompiled version.
The reason why this happens is that the public void func_180558_a(EntityFishHook p_180558_1_, double p_180558_2_, double p_180558_4_, double p_180558_6_, float p_180558_8_, float p_180558_9_) method of the net.minecraft.client.renderer.entity.RenderFish class uses the fields prevPosX, prevPosY and prevPosZ of the fishing hook to render the line. However until the fishing hook is not updated, its prevPosX, prevPosY and prevPosZ fields are all 0d. This could be solved by setting them all to the current coordinate values in the constructor. In general it might make sense to do this for all entities.