De-energization of transformers

I am power system engineer and currently working at national power control center of Pakistan. I have a question about de-energisationl/Energisation of Power transformer. In my control room following SOP's are followed.

For De-energisation
First LV side of transformer is de-energised then HV side is de-energized in case of permit of transformer

For Energisation
First HV side is energised then LV side is energised...

I want to know what is the logic behind this sop...Why it is necessary to follow these steps..
please help

Thanks in advance