VBA does what you tell it to. Nothing else. You are telling it to convert the activedocument range (all of it) to a table. You don't show what inserts the text, but look at this:
Sub ScratchMacro()
'A basic Word macro coded by Greg Maxey, http://gregmaxey.com/word_tips.html, 2/15/2018
Dim oRng As Range
Set oRng = ActiveDocument.Range
oRng.Collapse wdCollapseEnd
oRng.InsertBefore vbCr
oRng.Text = "Some inserted text" & vbTab & "Some more text"
oRng.ConvertToTable Separator:=wdSeparateByTabs
lbl_Exit:
Exit Sub
End Sub