Try this, you can easily adjust it to your actual sheet name and column to split.
Sub splitByColB()
Dim r As Range, i As Long, ar
Set r = Worksheets("Sheet1").Range("B999999").End(xlUp)
Do While r.row > 1
ar = Split(r.value, ",")
If UBound(ar) >= 0 Then r.value = ar(0)
For i = UBound(ar) To 1 Step -1
r.EntireRow.Copy
r.Offset(1).EntireRow.Insert
r.Offset(1).value = ar(i)
Next
Set r = r.Offset(-1)
Loop
End Sub