Macro runs when I change cells in totally different sheet. Why?

So I have a worksheet that has a lot of macros in it. I have just barely run into a problem that happens anytime a change a cell in another workbook that I have open. The second workbook I have open isn't even a macro enabled workbook. But for some reason it gives me an error and says I am trying to run this code.

Private Sub ProductSelection_Click()
Sheets("Transaction").Label2.Caption = Format(Sheets("Dummy").Range("LookupPrice").Value, "$###0.00")
End Sub

It gives me the runtime error 9. This code is supposed to run when I click on a combobox in my main workbook but it is running anytime I change something in my other simple workbook. Does anyone know why this happens and how to stop it? I have had a lot of similar problems in the past but I just dealt with it by only having one workbook open but in this case I will need both.

This is what I am trying now but it still doesn't work. When I declared the workbook within this private sub it worked great. I just want to make this a public function or whatever it takes so I can use the same text in all my macros without having to set it. This is what I have.

Public InvManageWB As Workbook
Public Sub workbookvariable()
    Dim InvManageWB As Workbook
    Set InvManageWB = Workbooks("Inventory Management DropBox.xlsm")
End Sub

Private Sub ProductSelection_Change()

InvManageWB.Sheets("Transaction").Label2.Caption = Format(InvManageWB.Sheets("Dummy").Range("LookupPrice").Value, "$###0.00")
End Sub

I figured it out. Here is my soluction

Public Function InvManageWB() As Workbook
    Set InvManageWB = Workbooks("Inventory Management DropBox.xlsm")
End Function

Private Sub ProductSelection_Change()
InvManageWB.Sheets("Transaction").Label2.Caption = Format(InvManageWB.Sheets("Dummy").Range("LookupPrice").Value, "$###0.00")
End Sub

As for why it was triggering the macro I still have no Idea.