Why models with parts welded together act crazy

[NinjoOnline]

As you can see from the video, one of the models is just flying around the place, while the other 2 models actually sit next to my player. The difference between the model flying around and the ones that aren’t is it has parts welded to the main part

The model in question

Server

function CreateDuck.OnServerInvoke(Player, Tier, Rarity)
	local PlayersDuck = Ducks['Tier ' .. Tier][Rarity]:Clone()
	PlayersDuck.Name = Tier .. ' - ' .. Rarity
	PlayersDuck.Parent = Player.Character
	
	for i,v in pairs(PlayersDuck:GetDescendants()) do
	    if v:IsA('BasePart') then
	        v:SetNetworkOwner(Player)
	    end
	end
		
	return PlayersDuck
end

Client

function follow(Pet, Pos, bg, bp)
	spawn(function()
		RunService.RenderStepped:Connect(function()		
			bg.CFrame = HumanoidRootPart.CFrame
			bp.Position = (HumanoidRootPart.CFrame * CFrame.new(Pos)).p - Vector3.new(0, HumanoidRootPart.Size.Y / 2, 0) + Vector3.new(0, Pet.Size.Y, 0)
			wait()
		end)
	end)
end

function renderDuck(ducks)
	local RotationOffset = 360/#ducks
	
	for i, v in pairs(ducks) do
		
		local PlayersDuck = CreateDuck:InvokeServer(v.Tier, v.Rarity)
		local Duck = PlayersDuck.Duck
		
		local bp = Instance.new('BodyPosition')
		local bg = Instance.new('BodyGyro')
				
		bg.D = 750
		bg.MaxTorque = Vector3.new(25000, 25000, 25000)
		bg.P = 5000
		
		bp.D = 400
		bp.MaxForce = Vector3.new(1250, 10000, 1250)
		bp.P = 2000
					
		bg.Parent = Duck
		bp.Parent = Duck
	
		Duck.Position = HumanoidRootPart.Position + Vector3.new(0, 0, 0)
		follow(Duck, GetPointOnCircle(5, RotationOffset*i), bg, bp)
	end
end