Inserting in the Middle
Formulas are always automatically expanded when you insert new rows and columns inside existing formulas.
Inserting a row above row 5 (i.e. in the middle) will cause the SUM() formula to automatically expand.
The same is true when you have formulas that refer to columns and you insert new columns inside existing formulas.
In these situatios the formulas will always expand (automatically) and there is no option to turn this off.
Check your Options
When you insert rows (or columns) at the end of a formula range the formulas (by default) will be expanded automatically.
Formulas will only expand automatically when you insert rows (or columns) at the end of a formula range when the following option is selected.
Check your (Tools > Options)(Edit tab, Extend data range formats and formulas) to ensure that this tag will be displayed.
Formulas will not be automatically expanded when you insert rows (or columns) at the start of a formula range.
Formulas will not be automatically expanded (or copied) when you paste data from the clipboard.